One more option is to use a micro servo in the engine compartment. I saw this setup a couple of years ago, and the RC'er said that it worked just fine. He put a think coating of RTV or silicon on the servo, except for the main shaft area, and fabricated a mount in a position that allowed him to do what he wanted with the throttle.

This will work, of course, if there is enough room in the engine compartment.

CGr.