RE: tuned pipe
Make your pipe adjustible .. after finding the right tuned-length with a decent prop . you can weld it again.
But I'm not a fan of that ... a different prop may need another tuned-length.
Here in Europe the proper tuned-length for a .21 is between 8.2 and 9.4".
Measured over the center-line from glowplug to exhaust belly-seam.