My O.S. .91 is constantly throwing ts muffler off.. (partly because of the home made header experiments for the tuned pipe)

Because I know this is likely... I leave the engine tuned a bit extra rich, and it doesn't lean out far enough to be harmful. No pump. If I blow the header AND am doing a high AOA maneuver... the engine dies. the muffler pressure is more for ensuring good fuel flow at all attitudes than it is for level flight ops.

You can run ANY model engine without muffler pressure.... It just may not do well at anything other than puttering around the trafic pattern.