I THINK they're all interchangeable although there is speculation that the cylinder or head (i dont recall which) is slightly shorter on the 86-88 year models. Still interchangable as far as i know but you'll have more slack in the cam chain. IIRC. You'd be better off matching years if you can or at least matching 86-88 models parts  rather than 95-present parts. Im pretty sure heads are interchangeable through all years but you'll want to get a head/headcover as a set as they're machined to match. Not sure if there's a difference in the cam chain sprockets between years.