This is a 127/100 gear on a Model C.
Your gear should go on the Banjo, more or less in place of, or in addition to, the 80 Tooth Idler. However there should be a spacer, behind the gearbox drive gear. This will need to be moved in front. The Twin gear rocker gear should engage the larger of the gears, then the Gearbox gear engages the smaller one. With your smaller gears you may need to add an idler to make things fit.
As noted you will not get an exact 25 TPI. Just changing the rocker gear or the gearbox input gear would have been better.