Here is a link to a rotax sight for ultra lights. slugs are slugs, cylinders are cylinders, no matter wether it flys, floats, ...or scuttles along over snow. http://www.theultralightplace.com/pistons.htm I would venture to say your problem is "detonation which always precedes pre ignition" (piston no.6 in pics) what causes it varies u may have had an oil injection check valve failure for that cylinder, did u check to make sure the oiler hose from oil pump is connected? I just rebuilt my '97 600 zrt top & bottom posted question earlier today on oil pump no reply yet
Air leak, or is it possible something broke and the piston is a result of a failure.
On a 3 banger the other 2 will try to push the bad cylinder and sometimes it is hard to tell until it is too late.
Crank seal? Have you blown a belt bad lately?