Hard to say MJD-he hasn't really given us a lot to go on. In terms of plastic deterioration-a lot I would suggest-depends on how the engine has been stored since he first ran it in the 90s. In a drawer, in the dark-fine, its probably nothing worse than gummy oil residue. On a mantelpiece, shelf or windowsill-exposed to sunlight and artificial light-different story altogether.....
And secondly he didn't say it was made then only that he bought it new back then-it might have been sitting in the shop or dealers for years. Anyway distortion/cracking of the carb housing is only one of a number of possibilities for his problem....

ChrisM