I believe your diagnosis is correct. The cam sprocket skipped a few teeth. Now the piston is touching an open valve when you rotate the crankshaft. Put the cam chain tensioner back in and tighten it up so there is tension on the chain. Rotate the engine backwards so you're not bearing on the valves while trying to break the cam sprocket loose. Let the foot peg hold the bottom ratchet while turning the ratchet on the cam sprocket bolt. It's tight, but it will come off. You'll have to re-time things when you go back together, but it's not that big of a deal. The sticky about base gasket replacement in the technical help part of the forum describes much of what you're doing and might be helpful to you. Good luck, and post up if you need any further help.