Most FFs will convert easil. If you start from a stick-and-tissue model, the control surfaces are generally defined by a spar. I double the spar with 2 pieces of wood, build the surfaces and cut the control surfaces from the assembly. Foam models ussually have the control surfaces embossed, and they can be cut free. On either type of construction, CF rods make good stiffeners and connectors (for the two elevator halves). Most rubber models are designed to be strong enough to hold the tension of a wound motor, and subtituting light R/C gear only requires adding a few peices to make equipment mounts.

The biggest issue I find is making a suitable motor mount; this often involves making a fire wall from ply and/or balsa, beam mounts, etc. Rubber models tend to have cavernous nose plugs to allow stretch winding, and sometimes these are a problem.