All the diver brakes on A36's were eventually locked closed because of the problem it poised with aileron control and buffeting it created for the wing. Its going to be simple to replicate as you need only place a plate between the rib at the level of the skin on the top and bottom. Attach an aluminum bell crank turned sideways and drive both top and bottom brakes from a single servo in the center of the wing. Why a single? Well...two might not load up equally and result in a problem of no aileron control.

The cowl alteration will leave you with an exposed two cycle or four cycle head unless you use an RCV and its probably the best to use because it affords more torque for the three blad prop at such a small diameter.