Warning! Some OEM parts are no longer available for the RFVC XR500 (83 & later models).
If the bike's been treated nicely, and ridden normally, they're pretty much bullet proof. Unfortunately, if someone has starved the thing of oil, or not changed the oil frequently enough, bad things have happened.
As on most RFVC Hondas, the only real weak spots are the center cam journal and the small end of the rod. In low oil/bad oil conditions, the center cam journal wears and drops metal down into the lower end. Also, the small end of the rod galls, and then stretches.
Both of these problems will cause engine noises. Anyone familiar with the normal sounds of an RFVC Honda can tell you if the bike you're looking at sounds okay or not. If it sounds good, then it's probably a good bike.
If there's funny sounds, don't buy it, even if it's cheap. Fixing a worn center cam journal is do-able, but you have to start with a new cam and cover -- not cheap.
If the rod is bad, there's no replacements available. Not from Honda, not from Niks, not from Hot Rods.
PJ