Do not attempt to turn the engine over. You could break the rings if they are rusted to the cylinder walls. Pull all plugs, add a cup or more of automatic transmission fluid (ATF ) or Marvel Mystery Oil (MMO) in each cylinder. Pull both valve covers & soak everything w/ MMO or ATF. Scrub it all down w/ a wire brush. Then, start looking for mouse/hornet nests in places like the tail pipe & carb breather hose. Wait at least 2 or 3 days before you do anything, then try & hand crank the engine. (w/o spark plugs) If it does not turn by hand, repeat everything you just did, ie, more MMO or ATF. If it still won’t turn by hand, remove the starter (see tip # 36) and use a BF screwdriver or jack handle on the ring gear teeth to try & turn it over.