Cop a harbor freight predator 212 and a torque converter. The centrifugal clutch on the torque converter engages when it hits a certain rpm so the engine can send plenty of power the second it starts spinning the the pinwheel. Might be a good idea to remove the governor to get some more power out of it. This will increase top speed allowed by the motor, which will let you use a lower gearing/smaller pinwheel size for more instant torque = faster speed with a shorter in run.
A dolly is a decent chassis cause it makes it easy to transport, and can be easily be bolted down. I also highly recommend rigging a bike brake lever for throttle input. Will let the operator stand out of the way of the frozen rope getting sent toward their head.