Since you have not mentioned the OS75AX or 95AX as possible substitutions for the Tower 75, I am guessing you are not really wanting to spend Saito 115 or 125 money right now. I know my budget does not really fit Saito.

.91 to 1.20 is the recommended range for four strokes on this plane and the Magnum 120 is about the same size and weight as the OS. To compensate for the weight of a Magnum 120, you may need to do some or all of the following
> Lose the nose wheel
> Move the engine as close to the firewall as you can
> Move the elevator and rudder servos to the tail area
> Add a servo on the bottom to steer the tail wheel
> Put the receiver pack as far back as possible
None of it is hard to do. It all takes time though and it means you have to do surgery on a brand new plane.