As a first step to unsticking the engine, you might try filling the spark plug holes (add 4 oz or so) of marvel mystery oil or ATF and letting it sit in there for a week or so. Then try turning it using a wrench on the crankshaft. This may free it up if it is just stuck from sitting for a long time. Worst case would be it got stuck because the PO (doofus) ran the oil dry or something like that which would be worse.
IF you do get it free, its highly likely that you would need to at least hone the cylinder and replace piston rings because there's likely to be a mark on the bore where the rings rusted to the bore. It may run, but compression is likely to be below normal.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville