A slightly alternative approach is to get two gears which DO give you the ratio you want and mount them outboard of existing change gears of smaller diameter (turn a two-size mandrel, mount, drill & tap three holes, then bore the new gears large enough to take the axle-fixing screws). You may have to do some playing about to find a combination that fits your existing gears and carrier travel.
It beats looking for that ratio in your particular pitch. For Minilathes in particular, Sherline makes an affordable 127/100 combination.