i was referring to a warped head.. compression would leak from the gap between the head and cylinder...
if you checked rings and things look good, it's something else. I guess the ring grooves and the lands are ok so the rings lay flat and can seal.
To measure ring wear, gap the rings.. install a ring at the top and/or bottom of the cylinder and measure it's gap .. if its over 0.03" or so, its getting worn..
Cylinder glazing is accumulated, burned oil residue .. a good scrubbing of the cylinder wall will remove it.
a decompression valve is a little cable-operated valve in some heads.. opening the valve allows the user to pedal-start the engine very easily .. when some cranking momentum is built up the valve is closed, compresion builds and the engine fires.
Lots of people strip head-bolt threads by not using a torque wrench.. i recommend you find a small pound-inch wrench for moped work.