Always easier to diagnose before stripping down rather than after. First thing I'd have done was listen at the inlet and exhaust ports whilst turning it over (easier if someone else is kicking it over). Any hissing sound whould mean valve leakage, which could be as simple as tappet clearance needing to be adjusted. Rings? Unlikely. Some rings need to be installed right-way up, and are marked 'top'. However, if these are fitted the wrong way round, there will still be reasonable compression. A broken ring (caused during fitting barrels) will usually show a vertical scratch mark in the bore. Again, there would still be some compression, as the other two rings will still be working. But you suggest there is little or no compression. Very odd. Most likely a valve issue, but more info required.