Turning the 8x6 Prop at anywhere near the same RPM as the electric motor is going to take at least a strong .15. The weight of the engines is going to be substantially more than the equivalent electric motors so you are going to end up with a balance issue. Adding a couple of tanks of fuel is only going to add to the cg problem. You might consider going to
www.allerc.com for a couple of Hyperion motors which are better than Rimfires and less expensive.
Question -- is it the motors that are trashed or the esc's? If it is the esc's you can get good escs from Hobbyking.