Handling and tuning TWO contra pistons and a needle valve is bad enough. Can you imagine trying to match up FOUR! ? ! ? ! ? You'd get it running just right just about the same time it ran out of fuel......
Dtarking, for RC flying I'd suggest an inside doubler strip of wood around the wing saddle opening to strengthen up that area.
Success or failure for electric power relies on knowing the final expected all up weight of the model. So giving us the airframe weight combined with RC gear as it sits now would aid us in giving you a better idea of a motor.
My thought is that Pond Skipper's lower end suggestion of 60 watts might be a little light. For sporty flying I would suggest that you're looking at 70 watts per lb as a nice target for power. And at a 45 inch span and seeing the pictures so far I'm going to guess that your final ready to fly weight will be more in the 2 pound to 2.5 lb range. So you'd want a 120 to 140 watt max power motor. And because this is a little more than Pond Skipper's suggestions I'd suggest a 850 to 1200mah 3S pack over a 2S pack.
I'm guessing that you're also pretty new to this stuff. So you'll also need to pick the Kv value of the motor to suit the model. The Kv value is sort of like the gear ratio. Higher Kv values spin smaller props faster. Lower Kv motors turn big props slowly. For a model of this sort you'd want an inbetween sort of motor to turn a roughly 9x5 or 9x6 prop.
Keeping it cheap I'd suggest the following stuff from Hobby King;
This one looks like a nice option to me. Running it at around 130 to 140 watts will keep it comfortably below it's max continuous rating of 173watts so it'll tend to run cooler and live longer. Don't forget the accessory pack for the mount and prop adapter. And get a couple of extra replacement motor shafts at the same time.
You'll find LOTS of other options. But if you look for motors with the same sort of maximum watts and max current and the same Kv value as this one they will all be roughly equivalent. It's OK to go with higher wattage and max current but don't alter the Kv value or you'll need to use something other than a 9 inch prop.
I'd run this motor on a 9x5 prop and a 3S (3 lipo cells in Series, hence the "3S") using a 25amp to 30 amp ESC. This means the ESC is over rated but that's OK. We do it that way so the ESC isn't pushed hard and ends up heating up. It's pretty common to use ESC's rated for 1.5x to 2x the expected maximum power draw for this reason.
Note that the power that the motor will pull is almost totally dependant on the prop. Need more power? Go up in diameter or pitch or both. Motor or battery running hot to the touch? Reduce the diameter, pitch or both to make the motor pull less power.
To test the motor to ensure it's not pulling too much power and thus will try to smoke the motor the normal method is to use a watt meter. But if you don't want to buy one you can do a quick and dirty test to make sure you're not too close to letting out the smoke. With everything set up ground run the motor at full throttle for a minute. Then shut it down and disconnect the pack. By that time check the motor for warmth over the next minute. It'll take roughly that long for the heat from the core windings to make it out through the magnets and outer bell. It will likely become quite warm. But if it gets anywhere near to anything like uncomfortable to hold then it's too hot and you need to go down a size on the prop.