Hi birchy
This is all good for a belt drive conversion. No machining off gears with the associated problems.

Can i make a suggestion here.

"However, having drawn the design to scale, it turns out that the motor would foul the top of the spindle. Whatever I do, I'll have to raise the motor above the spindle".

Mount the motor on the side of the head with the spindle at the top same as a 'Seig X3" Mill Drill.

Not the prettiest solution just an alternative.

Please keep posting on this thread as this is useful way to make a vast improvement over noisy and fragile gear drives.

Eric