The T/F giant 51 will fly quite well on a Zenoa G-62. This is the heaviest of the gassers for its' size and output. You may need the weight in the front for balance purposes.

If you don't need the weight, might I suggest a good, high output 3.2 ci engine. My personal choices are Brisson, BME, and Taurus.

For good performance purposes, try about 1 cubic inch for each 8 pounds of weight. That will be for the all up, ready to fly weight. I like to add a couple of imaginary pounds for general purposes.