Today i saw someone on the track with a front geardiff.
it's pretty easy to build. he grinded the teeth of the diffpulley.
Then he superglued the pulley of the normal balldiff on it. He also but some screws in it i believe. And to get it aligned correctly he used the spacers you can put in your uprights, between the bearings. Works really good he said, car looked really fast on the track.