Hi Mike, no idea exactly what diameter and no. of teeth we are talking about here, but an idea – make or have made brass or mild steel gears, split them into halves, clean the shaft, flux, and solder / sweat the gear together in place on the shaft. Use just the tiniest pieces of solder, just enough to wick some in, so it doesn't run into the teeth. If the gear to shaft bond is not good a drop of 609 loctite cylindrical retaining compound would likely bond it.
Depending on quantity scope of the project and your budget, it is very possible to have an injection mould insert set made to produce new gears in acetal resin, with 3 pin gates for hottest knit/weld lines in the plastic and thus minimal chance of splitting again.