Has redoing the heat treatment on the modified gears ever been done? There is certified heat treating near me, I've had a 4130 chromoly skid plate done. Wasn't cheap but a lot cheaper than getting my aluminum skid plates done...
Carburizing is the process used on gears to case harden the wearing surfaces while leaving the base material a little softer so the gear is tough and not brittle. The carburizing process penetrates the base metal but also builds up on all of the surfaces changing the dimension of everything it builds up on. The amount of build up is predictable so all of the original dimensions have to be made undersized.
Re-carburizing would make the gears not fit over the splined shafts and the mesh of the teeth would be too tight.