I freed my 350 by soaking the pistons with WD40 for 4 days(a new spray each day) and then bouncing carefully on the kickstarter while in 1st gear.(I'm only 160lbs) Do be gentle if you use this method! I'd hate to see more parts damaged by landing on the kicker too hard!
I'd recommend pulling the engine and going through it anyways. Once running, mine snapped a cam gear screw and bent all the valves. It cost me a new set of valves and a week of work for the shortcut. I would have bought the gaskets anyway...
