RE: more power for a Tower 40 trainer
In my tower trainer, I have a Magnum 52 4 stroke motor in it, and its the perfect fit for this trainer. I have an APC 11x7 prop on it, and there's more than enough power for this plane. Combine the power of the 4 stroke with the gas sipping motor, and you can have a good 15-20 min training flight, and no need to add extra weight on it either. All in all, a perfect fit for this trainer.
One minor thing to consider though.... you will have to re-drill the firewall and rotate the carb around to make this work, but its worth it.
Just my $.02 worth......