It's actually pretty simple to convert a glow airplane to electric. The first thing you generally want to do is decide what kind of flight performance you want out of that airplane. Since it is a sport airplane and your old timer friend may not be into the ballsy 3D kind of power we generally prefer, I am going to assume you are going after sporty performance. Great Planes/Electrifly has already taken a LOT of the guesswork out of selecting a power system and has designed electric motors with glow number designations. For example, the
Rimfire .55 42-60-480 outrunner will provide more than plenty of power and spirited performance for that airplane on a 5 cell battery. If you can fit a 6 cell inside of that fuselage you will see extremely strong vertical power (near 3D) so the motor allows for a lot of room to adjust performance based upon the pilot.
You already have a good speed controller as well, one with a built in BEC so there is no need for a separate receiver battery. You will want to experiment with different propeller sizes as well to adjust performance and give the maximum flight times. Star with a 13x10 electric propeller and adjust from there.
There is also a couple of different ways you can mount the motor. First is just bolting it directly to the firewall with the included "X" style mount. Second is to use a much stronger outrunner box from Great Planes. Either way will work, which you choose is up to you. It's not really difficult to convert an airplane to electric nowadays, especially since a lot of manufacturers are starting to designate glow motor equivalents with their outrunner motors.