Bob Allison
Alan,
I’m afraid the starting point has got to be careful measurement of outer diameters and tooth depth (gears and worms). BUT… the trick then is to arrange for the housing of the layshaft bearings to be clamped with bolts in slotted holes, which will allow adjustment up and down. Once you have found the sweet spot, you can solder it all up solid and remove the bolts.
Regards,
Bob