If you are able to build, or have access to someone that can build electronics for you, you can build Dave's controller, otherwise I can send you one (would have to check the shipping cost, but probably around 30 Euro total incl shipping).

That small piece of electronics needs to be hooked up to the solenoid.
You will need a TX with decent programmability, like OpenTX or comparable.
A separate channel slaved to the throttle, programmed with a multipoint curve, controlling the solenoid, allows you to tune the engine over the entire throttle range.

That is it in a nutshell.