On your tank pressure question, on some of my planes I run the pressure line from the tank to just behind the prop. I take a fuel filter that screws together, unscrew it, take half put it in the line and mount it solid just behind the prop off of the engine mount. I put copper tube in it to make it stiff and give me a way to mount it. The ram air from the prop pressurizes the tank. I have done that on quite a few pattern birds with large tanks and pipes and have never had one lean in climbs and stuff. It works really well. Might try that on the one with the broken muffler.