If you're dead set on using a 91FX, then go ahead and build the Mojo 60. The 91FX will still give you plenty of power for basic 3D flying such as harriers, hovers, and torque rolls. Later on it might be worth experimenting with a Tower muffler, Ultrathrust, or a Mousse Can Pipe, but I'd start out with the stock muffler to make sure the engine throttles correctly for learning the basics. When it comes to 3D flying, a good throttling engine is far more valuable than raw top end power.