I am fitting a Sakura zero gear diff to mine, waiting on 40t gear to arrive, have made this changes so far, sounds a bit much but it fits nicely.
Changed plastic gears with xRay NT1 metal ones
Changed outdrives with short Kyosho RRR V1, Sakura's are too small.
Use the supplied ball bearings, they are plenty shims in Sakura's set to fit the width, the kit is cheap, I had the Kyosho outdrive and I bought the NT1 gears.
The gear is on the way and should land this week, so as soon as it arrive I will finish it and try it.
Mr Fit: You can fit front and/or rear as they are the same size.