RE: retrofittin retractable gear
You can get a set of these, and a micro servo with pump for the system, you'll also need to add plywood rib doublers to the ribs where you mount the retracts, usually out about 4th or 5th bay on the wing. Notch the doublers to accept 1/4" ply rails to mount the gear, after it's sheeted, you can go back and make the openings for the wire and the wheels and clearence the ribs inboard for the wheels. It helps to line the wheelwell with 1/64 ply, this adds streingth back to the area clearenced for the wheel. If your going to make gear doors, you can layup a surface mold with glass over wax paper or clear backer from monokote under it, laying between the glass and the surface. Then you can mold flush geardooors the exact shape as the wing surface. I usually only do that for my warbirds thou. The koas is a great flying plane, just keep in mind, there's very little room inside it for an air tank, retract servo&control valve. My 60 sized TF P-47 is cramped for room with the retract tank, valve, servo, tail cylinder, geardoor servos, the retract system is going to be really heavy compaired to a simple mechanical retracts, less to mount, just the retracts and a retract servo.
[link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJZD2&P=0[/link]
These would be better due to weight and room required to install them.
[link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXUY32&P=0[/link]