It is very comon for the valves to stick on these motors, espeacially if one was cranked with old gas in the system.

If you suspect stuck valves, pull the valve cover and rocker assembly. This will eliminate the valves as suspect.

If it is still stuck it is likely the usual piston rings rusted to the cylinder walls. Some diesel fuel squirted into the cylinders through the spark plug hole might help free it. I have had success using ATF. Let it sit for several days. Try spinning the motor both ways untill it will spin freely for a couple revolutions.

If you still can't budge it, it's time to pull the head and have a look inside. There are spy camera tools availabe that might let you look into the cylinders through the spark plug hole without pulling the head. Harbor freight has one that works well according to a trusted source and I have seen one recently at Costco.