Hmmmmm. Any ideas where the "hissing" is coming from?

Have you stripped the engine down to inspect the state of the piston / sleeve?

I haven't yet reached the point where a rebuild is necessary, so I can't really advise you on how to tell when one's needed, but if you have no/little compression then I'd say you're there

I know that with mine it's VERY hard to turn the flywheel manually through a complete cycle - give yours a try...