Hi!
The first thing you should do is to replace the tank and get a the best there is: a Tettra "Bubbleless tank! 12-15 oz depending on what time you want to fly. Second best is to use a " Uni-flow" set up in an ordinary R/C tank.
I would use use 5% nitro and 20% oil (Castor or a synthetic or a mix). For this fuel I recommend a Rossi 5 or 6 glow plug when using a pipe. Remember that using a pipe makes the compression go up as a pipe acts like a compressor on a model engine, thus requiring lower nitro contents to work well

As for props there are few that can rival RAM (British made) and APC! 12x8 or 12x9 is just fine if you want speed. Even smaller props with more pitch could be tried if the plane is small. 14x6, 14x8 and 15x4W APC is for large lumbering airplanes like scale type warbirds like P-50 Mustangs and semi scale aerobatics Velox and Cap types.