Although duplicating China's economic success is probably unlikely (the Chinese still had actual, money-hungry peasants rather than a rural proletariat scarcely less lackadaisical than the factory workers, and small-scale capitalism hung on in China in ways not duplicated by the USSR, _and_ the Soviets have a colossal monkey on their back in the form of all the people and resources tied up with ineffective industries) I think you probably could have economic reforms that could help.
Work on the infrastructure, one of the worst problems with food production was transportation bottlenecks that left stuff rotting in the fields or silos: move collective farms over to a profit basis (and use the KGB harshly agaisnt those who will try to resist such innovations in protection of their own applecarts), allow people to start up their own small businesses, genuine tranfers of factory ownership to employees, improvements in the overspecialized education system (a degree in ballbearings?? Really, comrade?) And cut the massively uneccessary military budget - but then you have to find jobs for all the soldiers. Avoid, or at least manage the emergence of something like OTL's "Robber barons", and crack down harshly on those who try to smuggle new wealth abroad.
Of course, all this needs a great deal of _power_. A longer-lived ruthless SOB like Andropov might have had a better chance of consolidating power in his hands sufficiently to push through serious reforms - and, more importantly, have them actually applied at "ground level." Any would be reformer is faced with the problem of massive resistance to change by all those who benefit from the ways things are, and fear, justifiably in many cases, that changing things will make things worse.
It strikes me that it's hard to fix the USSR after Brezhnev - he was, in his own way, a skilled politician, and what he wanted internally was stability, not Sweeping Revolutionary Change. Corruption, even massive corruption, was simply one of the necessary oils needed to grease the system, and the Appartchniks had to be kept happy. The oil helped, too, and although economic growth didn't really stall until his last years, under him the "system" petrified in place to the extent heavy chisel work would be needed for any major changes.
Do I think it's impossible for the USSR to keep going after 1980? No, I think given enough willpower to hold the thing together on the part of the leadership and some possible economic reforms, the system could still limp along till today: if the concentration camp and the military prison remain open, if the army can still be (mostly) relied on, the USSR can drag it's way forward as long as things don't reach the-shooting-participants -mass-food-riots stage of things. Heck, they might even achieve economic reforms on a piecemeal manner to reach the glorious standard of living currently enjoyed by Poland.
Of course, this is hardly guaranteed: you need strong leadership with a good idea of what the hell they're doing, and the odds are that you get either one or zero of two, leading to a slower and messier disintegration of the USSR in the 90's. Eevn if we avoid such a debacle, there is also the problem of Eastern Europe. There is an excellent chance that Ceausescu still gets killed by his subordinates, although as long as they send his head in a box with a note explaining their eternal devotion to the USSR and Marxism-Leninism, the Soviets probably don't respond. However, Poland is likely to explode again, perhaps by the early 90's, and the Soviets may have no choice but to intervene or lose one of the keystones of the Warsaw Pact.
Of course, other things will be happening in the meantime. Will Mandela be released and negociations begin in a world where the African National Congress is still seen as tied to a dangerous and non-reforming USSR?
Will the Soviets be more sucessful than the US in deterring Iraq from invading Kuwait, and if he still does, will they back him (the Soviets, too, like the notion of higher oil prices) or will they insist on an equal role for their military in their expulsion of Iraq from K.?
How many cheap knockoffs of "Red Storm Rising will be written?
Will the Russians hold their noses and help fellow Orthodox Slav Milosevic hold Yugoslavia, perhaps in exchange for basing rights on the Adriatic?
Without a Soviet collapse and arms control deals stalled by the Soviet's desires to see the "star wars program" - in their view a support system for a first-strike strategy, not a defense vs. sneak attack - taken off the table, Reagan's presidency is not burnished quite so brightly.
Hmm - Soviet invasion of a Poland in revolt, 1993: massive mess, given that OTLs invasion of Czechoslovakia in 1968 turned into a logistical nightmare. Highly unpopular at home and abroad: other revolts (E. Germany, Czechs?), spread? Or is the Soviet invasion brutal enough to temporarily convince people to keep their heads down? In any event, it does not play well in Peoria, and the US finger-wags most vigorously and perhaps imposes some sort of embargo (those agricultural and infrastructure reforms could really come in handy here).
Bruce