First thing is to check that all of the pushrods are on their rockers & the rockers are working the valves
If the inlet don't open you don't get any compression.
Once you are sure the valves are working then pour some heavy oil down each plug hole & crank the engine with some rag held tight in the plug holes
Do this for a minute or so the take the rag away crank a few more times then do your compression check
Running out of oil can leave the bores dry and a dry bore will have little compression.
It takes 5/8 of SFA alloy to smeer around a ring grove to stop the rings making a seal against the bore.
OTOH it only take a little more to score the bore top to bottom.
If it were me I would grab a bore-o-scope and have a look inside they cylinders.
After that it would be off with the muffler & carb then with one cylinder on tdc compression shine a strong light down the inlet & the exhaust port while looking for light down the plug hole
Do it on both cylinders.
Any light = burned or hanging valves
Then there is the head gasket.
Overheating caused by low oil causes a touch more expansion in the barrel & head which squeezes the head gasket a touch thinner so when you start up again, it blows out in no time flat so again no compression.
Think that just about covers all bases