First off order a crank socket. Then remove the exhaust manifolds and risers (elbows). Those exhaust castings are very suspect, likely rotted internally in a way that kills motors. Remove the spark plugs, oil the cylinders, and let them sit a bit, maybe overnight.
Take the crank socket and bar over the motor, with a breaker bar or long ratchet.
Once you believe it has a functional rotating assembly compression test it.
Then you can pressure test the cooling system, if it's closed cooling use a car pressure tester and some mechanics line clamps. If it's raw water cooled you test by line clamping the two or 4 heater type hoses that lead to the exhaust. Then pull the inlet hose off the cooler under the port exhaust manifold. That's the engine's cooling supply hose. Rig up something to pressurize it to around 15 psi.
I used a cylindrical plug with a piece of inner tube containing a valve. I cut the tube in a circle around the tube valve, crammed the valve in the end of the plug. Then shoved them into the hose with the inner tube section surrounding the outside of the plug but inside the hose. Clamped it and voila. Air valve to the cooling system.
