420cc Honda clone (190F) generator engine always kicks back when trying to start

This started happening randomly, initially it would kickback 2-4 times before it would eventually turn over and start running, but it ran rough and hot, and started dieseling when shut down

However, after giving it a shim head gasket to bump up compression (because I planned on running it on propane only) it would never stop kicking back and i cant get it to start.

I've replaced the magneto coil, spark plug, ignition/starter switch, crankshaft (due to potential gear slip on the shaft), and camshaft, to no result.

I've checked the valve lash, the timing key, and magneto coil lash, and they are all good.

I've tried retarding the ignition timing using a custom timing key, and adjusting the air/fuel ratio to no result.

It's behaving as if it has a massive ignition timing advance, with the kickback and the past rough running, overheating, and dieseling. Despite having a stock timing key, coil, and flywheel.