those kimbrough rings were meant to keep diff balls cleaner in a different era of tracks and racing. they do not protect teeth on gears.
there is no rule that you have to run 64P gears on a 12th car either, highly recommended for racing.
on a 2011 plazma ra and the 2015 2.0 version spurs gears and diffs are standard like most 12th cars now. US diff gears work on it, it would be easy to fit those car with 48P and 32P gears for unprepped asphalt, streets, etc.