It's not uncommon for four-stroke engines to lose their valve seal if they've been sitting around for a while. Assorted crud can get caught in the valve seat area and sometimes the valves can get a bit sticky.

Mount the engine in a test stand and put some fuel in and prime it, wind her over and start her up. 95% of the time, this should restore compression. If compression is still dodgy, check the valve clearances once everything's freed up again. They should be set to between .0015 - .004 inches.