The lighter the engine, in the 50cc class that is, the better the performance. Even if you have to temper the weight by the price a heavier engine will still provide excellent performance.

I've seen the plane work with a Brison 3.2, a DA 50, a BME 50, and a G-62. None of the engines were a dissapointment, so go with what you can squeeze by the wife of significant other.