Remember to have a torch nearby. heating the bolts will expand the aluminum and free them.
Start in sections. I dont think its in the cylinder/piston section as you have good compression. Take the carbs off as a whole unit so its not a mess latter. Take the intake off and inspect the reed valves. Last resort is spliting the engine and inspecting the crank and bearings both upper and lower. It's a relatively easy pump, driven by crankcase pressure. Email me directly at
Rolrodjr@aol.com I've been there buddy....