If the rust has not agressed the heat treatment of the gears, it will work perfectly again.
Just it needs to be dismounted and putted in a tribologic superfinishing machine for a night and it probably will work again.
This machine looks like a cup fullof strange stones and vibrating at high frequency. Some kind of acid is in the bath as well.
You shall try it first.
Ciao
Wanni
Check
roesler.com there are those machines shown on the web