If you have time for a compression test then you would have time for a leakdown. A leakdown for vacuum will tell you a lot more than a compression test. If you are using a vacuum source stronger than lung power (vacuum syringe) you can suck the piston up with the vacuum syringe if the wall is wetted and the rings are above the roof of the exhause port. If the rings leak you will not be capable to draw the piston up.
You can also check for the carter gaskets and seals condition with a test at BDC with intake and exhause blocked off.
Otherwise the thumb on the sparkplug hole test will suffice for the top end compression check. Even rotating over the engine by hand you would feel resistance of compression if the rings were good.
Old engine that has been sitting usually needs gasket-seals-rings within a year of purchase, 40+ year sold is 40+ years old, just look at how cracked and dead half a century old tires look - the seals are likely going to be in the same dried and cracked condition.