Theoretically, you can have the Ki-61 (call it Ki-61 KAI) fitted with a 1250HP Ha-112 engine in 1942, as i understand it it this is the Army version of Navy's 1300HP Kinsei-50 series. Speed i would estimate it in the range of 570kph. Fitted with similar armament to Ki-61, the aircraft will be lighter and more maneuverable than Ki-61, yet still maintaining the high-dive speed, sturdiness and protection. Could make a difference in New Guinea, as it would be much more reliable than the regular Ha-40 fitted Ki-61. The 1500HP Ha-112-II engine was already flying in prototype form in late 1942, if they divert more resources to it from the Ha-40 engine which have proven problematic, it might enter series production in 1943. So you can have something like Ki-100 entering production (590kph max speed) by end 1943, rather than early 1945. Increased Ha-112 production could also lead to something like the Nakajima Ki-116 produced in parallel to Ki-84, with the also problematic Ha-45. This could replace the historical Ki-43-III production. Ki-116 would be head and shoulders above Ki-43-III, much faster and stronger built, better protected, 4 guns instead of two.
Finally, if someone is really thinking you could have something like the A6M8 fitted with Kinsei-60 series engines (Army Ha-112-II) flying in prototype form by mid 1943. If one can assume they would still keep it as light as possible, armed with 2x 13mm MGs and 2x 20mm cannons, and it wouldn't have the armour and self-sealing tanks historically present on A6M8 (and the bomb racks), then it will be a lighter machine and presumably a bit faster (say 580-585kph). You could have significant numbers of these by spring 1944. Would they make a difference? Who knows, but certainly would help.
All in all if this were to have happened the air war for the US would have been significantly more costly in 1943-45.