The manpower and resource situation of the Soviet Union precluded such.
Food:
The Bread of Affliction: The Food Supply in the USSR during World War II, by William Moskoff -
"The central fact behind the increased importance of the collective farm market was the drastic drop in food production, especially in 1942 and 1943, and the diminished proportion that went to the civilians. In 1943 overall agricultural production was only 38 percent of the 1940 level. In 1943, however, the Red Army began to recapture agricultural areas of the Ukraine, Belorussia, and the Caucasus and by the next year, 1944, agricultural output had risen to 54 percent of the 1940 level. Not surprisingly, the collapse of the food economy led to astonishing increases in prices. The most rapid
rate [Emphasis by author] of increase in prices took place in 1942 and began to taper off in mid-1943."
The Soviet Economy and the Red Army, 1930-1945, by Walter Scott Dunn
-
"By November of 1941, 47% of Soviet cropland was in German hands. The Germans had 38% of the grain farmland, 84% of the sugar land, 38% of the area devoted to beef and dairy cattle, and 60% of the land used to produce hogs. The Russians turned to the east and brought more land into cultivation. In the fall of 1941, the autumn and winter crops increased sharply in the eastern area. But despite all efforts, farm yields dropped from 95.5 million tons of grain in 1940 to 29.7 million tons in 1942. Production of cattle and horses dropped to less than half of prewar levels and hogs to one fifth. By 1942, meat and dairy production shrank to half the 1940 total and sugar to only 5%. Farm production in 1942 and 1943 dropped to 38% and 37% of 1940 totals."
Food production in 1943 was actually lower than in 1942 slightly, due to the failure of the potato crop in the Urals, despite the recovery of the North Caucasus and the East Bank Ukraine; the recovery of these territories was the only reason why mass starvation didn't totally breakout at that point even though mass death was being reported, particularly among factory workers. Rations were cut at the lowest possible amount and the only thing keeping the Red Army fed at this juncture was American Lend Lease,
as told according to Hunger and War: Food Provisioning in the Soviet Union During World War II. In the event of separate peace, those American supplies will be lost and the Soviet state will be unlikely to survive the coming winter without Ukraine and said supplies.
Production:
A review of production vis-a-vis the Soviet Union and the Reich will find that
the Reich consistently outproduced the USSR, which makes sense give the disparities of steel and coal production between the two. More critically for our purposes, however, is the specific production totals of the Soviets, which I shall highlight by selecting the year 1943:
Germany/USSR
1943:
Tanks and SP guns: 12,063 / 24,092
Armored cars: 806 / 1,820
Half-tracks: 16,964 / 0
Trucks: 109,483 / 45,545
Cars: 34,478 / 2,546
Locomotives: 5,243 / 43
Train cars: 66,263 / 108
As you can see here, while the Soviets maintained an advantage in things like tanks and armored cars, their production of trucks, half-tracks, cars, locomotives, and train cars was negligible or non-existent. This can be explained by Lend Lease, which in the event of a separate peace the Soviets will be, again, shorted. Given that it would take a long time to retool their machines tools, that 25% of said machine tools during the war were of American origin (Whole factories were shipped to the USSR), and the limits of their steel and coal output, it is doubtful they could ever meet their material needs especially given
that IOTL the Germans managed to destroy the entirety of their production in 1943 and nearly did so again in 1944.
Raw Materials:
The loss of the Ukraine and other occupied areas had engendered shortages of coal (The Donbass was home to roughly 60% of Soviet output by itself), aluminum (Main Soviet facility was along the Dnieper, about 60-80% of production), iron ore (60% of production), steel (50% of production), electric power (30% of output), manganese ore (30% of production), and nickel (30% of production). Overall output of the machinery and metal goods sector had fallen by 40%. In addition, the USSR was also unable to meet the demand for copper, tin, zinc, lead, aluminum, and nickel with remaining sources; Lend Lease was sufficient to meet all of these demands except for aluminum and nickel. Antimony, tungsten, cobalt, vanadium, molybdenum, tin, and magnesium were also almost entirely lacking. In short, there is no way, even presuming the industrial base was there for it, that the USSR could resume the war from a lack of raw materials alone.
Manpower:
Two reports that I am aware of were prepared for Stalin on the manpower situation over the course of the conflict, the first in September of 1942 and
the second in February, 1943. The
second report is summarized here:
The second report from Tschadenko submitted on 14 February 1943, situation as of 1.1.43:
In Army, Navy, NKVD troops - 10 947 000 (*) men plus 851 000 in hospitals
class of 1925 in process of call-up - 817 000
recruits transferred to industry work - 2 541 000
discharged or on leave for medical reasons - 982 000
nationalities exempt from military service - 250 000
irrevocable causalities - 5 950 000
lost on occupied territory - 5 631 000 + 965 000 (classes of 1924-1925)
A remainder (not called up yet) - 3 724 576 (of them about 2.5 million reserved in economy)
GULAG, NKPS troops and other minor military seem to be forgotten
(*) Breakdown of military:
On the front - 6 191 350 men
Far East - 1 131 696
internal military districts - 1 932 995 (including 1 422 659 in replacement, training units, and military schools)
others - 744 901 (reserves, units in transfer, airborne forces, separate air force and air defense elements)
Total Red Army - 10 000 942
About 946 000 in Navy and NKVD (calculated from the difference)
Those data were not necessary accurate and must contain some guesstimates and double counting, still they provide the general idea.
So in general, the Soviets had about ~3.7 Million left to call up, of whom 2.5 Million were needed to keep the economy going and most of the rest were Central Asian natives, whom were not exactly ideal to call up due to reliability issues. Even if they do call them up, that's just an additional 1.2 Million, which for reference is essentially the casualties incurred at Kursk and Smolensk in 1943 alone.