John,
I don't know what scale you are going with but the helicopter main gear that I used on my crane might be in the ballpark for you mixer. And, I know you can get different sizes for the different model copters, and they are not that expensive.
This one fits a T Rex 500.
If you have a space under the frame or in the cab I would try and use a servo converted to continuous rotation and then make a fles shaft to run up to the gear and put a pinion on the end of the shaft.
Jim