The muffler is easily fixed with a bit of JB weld on one of the to parts. After curing simply sand to a snug fit and it will stay in place, this will also ensure an even pressure from the pressure feed. A quicker solution is to JB weld the things together permanently...
I've also put some JB weld on the muffler section facing the engine to get a better seal.
I've spent much less time on my AP engine then I have on the TT07, and I shouldn't even mention the CS engine...
If you have a good piston and liner in your AP there can only be a minor fix to have it going well, I would think. I would recommend full castor in the fuel, I don't know what the hobbico has in it. I'm now running 20%castor and 10%Nitro, needle setting is around two turns open and then it runs really rich. I would trust the actual turns though, if it is too lean simply undo the needle more!
When the needle falls of you'll know that you have fuel blockage... (been there myself)
What is the weight of the plane?