Get the 3-gear set from Cermark made for F16 propjet. It can be made to fit nicely if you have not built the gear areas on your wings yet. IT's only 96 bucks for complete set. Spring air will work but twice the cost.
Please don't put a 4 stroke motor on a propjet. That's crime against humanity.
You want a motor with high pitch sound. Besides, 4 stroke motor is too heavy and looks funny up front of that narrow area. Use your OS 91 fx and add Jettstream muffler and you're ready to hit some high speed passes.