I know that engine, or at least the ASP version, which I believe is identical. I've had two and both have had or quickly developed considerable slack in the bottom-end clearance to the extent that you can hear a click when turning the engine over. (never had any similar - or in fact any problem with other ASPs in that respect)

When running, if that clearance is too high, the click (knock) will send shock waves that may "rattle" the plug element causing rapid damage.

Another possibility, but less likely, is the out of balance prop you mention.

Check your big-end clearance - turn the engine over and feel for clearance as the piston goes over top dead centre. If you've made all the usual checks (fuel, head clearance etc) then that would be my bet.