a .61 is 10 times the size use for a 1/2A plane. it should OBVIOUSLY not fit.
The Norvel .062 or .074 would be more approprate for a 1/2A Cap 21. (maybe electric power it...)

Since it seems you are going to train yourself... you better buy stock in a balsa forest. There are no good ARF's that I know of for self-teaching in a small space. (There's not much thats good for small space learning even with help...)

Cheapest, easiest... (if you're going to break em... make em cheap.)
Go to Wal-Mart or similar places, get one of the 4 1/2 ft span foam gliders.
Find out where it balances, and mark it.
Test glide a few times, and glue the tailplanes in place when ts set up for a lng, smooth straight glide.
Install 2 servos behind the wing, run pushrods back to the tailplanes. ADD 1/4X1 inch tapered TE stock balsa rudder and elevator. (you can even hinge then with rubber bands looped in figure-8 fashion, the crossover being between the fixed and moving surfaces) Install control horns...
place the reciever and battery on the forward fuselage, mve them as needed to get the plane to ballance as it did originally.(that's why its marked.)

You now have a 2 channel glider.

Later, you can add the Cox electric motor to the nose of another of these... You'll want to reinforce the powered version though...