At this point in your struggle, applying a load of heat to the valve joint won't hurt. It'll fry the seals but new ones are needed anyway. For the joint seal SiA128 I've used a Buna O-ring from my box-o'-stuff that seemed suitable in size, worked fine. The exhaust seal is a flat one, looks like urethane, and an O-ring wouldn't likely work here. Pretty sure I once cut one from 90 durometer urethane sheet, but didn't record the dimensions.

Don R.