The most common problem that I can recall is damaged teeth on the sub-port drums from owners letting the service intervals for the top-end go too long. If you don't service the power valves regularly, they tend to carbon up and seize, damaging the drum teeth and the timing rod. If you ride the bike and it seems to have really soft power at low rpms, there's a good chance that the drum valves aren't functioning correctly.
Pay very close attention to the motor mounts, especially the bottom ones.
I looked it up, 91 appears to be the first year of the perimeter frame, I didn't think that was until 92. Anyways, I don't think the perimeter frames had the same cracking problems as the 90 and earlier frames.