I will be rebuilding my '98 Fe501 next month and I just got thru building a '98 600. All parts including piston were available within three days. The piston is a bit pricey, but it is only the second one for the 600 and the 501 is still running strong on only its second piston since i bought the bike new . Don't know what all the fuss is about. All you have to do is perform regular (intense) maintainance and ride the snot out of the bike . Dan at MotoXotica can get just about anything to keep it running. Not too many aftermarket parts exist, but the bikes don't need much anyway.
My '98 bergs are 17 years newer than the last bike I raced. Even so they are a little out of date, but they are still an excellent play bike.