No Compression

Could be a lot of things, you know "why it was parked?"

Could indeed be head/ gasket problems

Could be stuck bent or burnt valves

Could be rust in the cylinders / rings/ valves AKA a combination of problems.

Very first thing I'd try is to squirt a little oil down each plug hole and re-test compression

MAKE SURE your compression tester is not leaking. If it's a "hold in by hand" deal, I'd find a screw in one, not that hard to find at garage sales, CL, or even new, not that expensive

Looking at the valves, just use some logic. If there is slack in some rockers and not others --then those are stuck open.

If you have a compressor, "rig" a fitting to inject air in the cylinders. You need to do this at TDC for a given cylinder, and look and listen for air out the carb throat (intake valve) out the exhaust (exhaust valve) or bubbles in the coolant with the cap off

N' yah. some sort of aggessive solvent, a couple cans of spray carb cleaner, paint thinner, etc, would probably be helpful. Do NOT turn the engine over very much with stuff like that in there. Add oil to lube the rings first.