Hi,

I am currently considering the same option as you are. Taking a Hobby Lobby Mini Telemaster (cost is about $25 US) and bolting on a Norvel Big Mig 61. This airplane has a very light wing loading so you do not need anything more than a Norvel 74 or a 10 size engine at most.

Of course you can also bolt on a 15. This large of an engine is not going to be a problem as long as the airplane balances. Keep in mind that if you convert to glow like a Big Mig 61, the plane will actually be a few ounces lighter than with a Speed 400 or similar.
If you have to add weight to the tail to balance the plane, your engine is too large or too heavy and performance will suffer. Better to move the engine closer to the C of G, although harder to do once the plane is built. Do not mount the engine as far forward on the engine mount is another option, if possible.

I use a Hitec 81 for throttle and Hitec 85 servos for rudder and elevator. Will be less than 20 ounces flying weight. Hobby Lobby are quoting 20 ounces flying weight with a 2 ounce electric motor, 6 ounces of batteries and 1 ounce for the electric speed controller.

9 ounces per square foot equals awesome flying.

Remember that when converting to glow, the front end of the fuselage needs to be beefed up to handle the vibration of an IC engine.
The front end, including the fuel tank compartment needs to be fuel proofed using thin CA. Seal the area where the wings sits on the fuselage using a thin layer of silicone. To do this, put a bead of silicone on the wing seating contact points, wrap the wing in Saran wrap, mount the wing and let the silicone cure. 24 hours later take off the wing and remove the saran wrap.

Remember, Glowfast and Crashlightly.