Who makes a 15x5?

Yes, if you use 91 FX, the stock muffler is not only heavy, but also quiet (performance-wise). Someone has good luck with 14x5N. This prop really unwinds the 91 FX (12,500 rpm?).

I do have a proposal to use Magnum 120 4-stroke with APC 16x4W or Zinger 16x5. Problem would be weight and size of this 120. You might as well not cowl the 120, because its valve cover will stick out of the bottom, and will force engine mounts outside the cowl on both sides. With the extra weight, you have to put battery pack under stab to balance at 6". But when it's all said and done, you'll have a rocket.