Do a search on "electric conversion". Basically you want to cut away enough of the engine bearers so that your chosen motor will fit, then figure out how to mount the motor. I think I'd get an electric motor mount from Bob Hunt (or shamelessly copy his) and replace the nose ring with the motor mount.
Make sure the motor's mounted firmly -- at the Northwest Regionals this year my plane was innocently sitting in the pits when someone's motor landed on its wing.
Tim there is nothing innocent about that airplane!
Alan,
a couple notes,, I fly a scratch built IMpact electric powered ( there is a thread on here if you search for my name), I can suggest that you make sure you have enough room to get the CG about an inch forward of stock glow position. I havernt touched an Impact ARC so I cant really be specific,,,
Make sure the battery is centered on the wing centerline vertically,, make sure you have enough room for the ESC to have cooing airflow over it,
What are you planing finish wise?
You can find the particulars of my setup in my thread,,
basically, eflight 32, castle Ice lite 75, Hubin timer, APC 13x4.5 P ( F2b) zIPPY 2700 MAH 6s PACK
Let me know if I can help more specifically,, target 68 ounces ready to fly mine is 72 and it shows some times