The plan here is a zero-cost re-purposing. The RC engine is no problem. Who doesn't have a stash of OS throttles pulled of engines for racing & stunt?
I have converted three 2-line planes to throttle control, retrofitted two from 3-line to 2.4 and built two all-new 2.4 throttle control models.
I count the battery, receiver and servos as "free" since I bought a supply in bulk off Ebay.
The steps are: install battery, remove racing equipment, move leadouts back, clean off old covering, reattach wing, line up throttle servo, make receiver box, make new landing gear, recover & paint, install hook system.
I went with the four AAA throwaway batteries. I use a box that includes the switch. No need to buy connectors. Use the wires from broken or fouled servos. Servos are the new glow plugs. The weight comes out right for tip weight.