Opinion from someone who has been in similar circumstance:
Long sitting engine - trying to revive.
(I am no expert. Everything I'm about to say I learned here.)
Compression gauge is cheap way to start. Doing a "wet" test (use forum search) after the initial will help narrow whether it's a top end or bottom end issue.
If it's a bottom end compression loss, don't immediately assume rebuild. Might be rings stuck from it's long nap. Do a forum search for "Kroil" and look for Hotrod Lincoln's procedure.
A basic head rebuild wouldn't be too bad, assuming yours isn't cracked.
