1986-1988 Cylinders are interchangable. Look for one that is complete with the KIPS valves, covers, head studs, etc. If not, someone in the FOR SALE section of this site has a set of KIPS valves. These are VERY hard to find, grab them when you can.
Try NOT to run a sleeve in a cylinder. The factory Nikasil cylinders are best.
If you need the valve bushings and a valve actuator rack-shaft, I have spares.
1986-1988 heads have different part numbers, but are basically the same.
Without buying a bike with a cylinder, you will have to buy may different parts, KIPS cover, seal housing, stop bolt, reed cage, etc. Way too many parts to expect to find new OEM. Much of this stuff is no longer avialable.
I would look for a parts bike on Craigslist, etc. or it will get expensive quick.
Use a 1989-up piston, works great, Dennis Kirk sells Pro-X and Wiseco sets.
Use 1986-1988 gasket set.
You can use a 1989-up FMF Gnarley pipe with minor adjustments.
You should try to find a late model KX500 (2001+ ??) Kiehin PWK carburetor, it fits perfect and runs better than the 1986 Mikuni.