Well the original motor is probably a Baldor DC commutator type as used in other Denford machines - like my little Novamill. Max spindle speed is 5000rpm which is OK though I'd like higher for engraving. Power approx 0.5hp I think. The mills don't have reduction pulleys for more torque at low speed. OTH DC motors have good low speed torque. I think you should at least try replacing the original motor/spindle and see how well that works - the Novamill is fine for cutting steel with this type of motor, the triac should be too. If you don't have the drive I can recommend KB Electronic types.

If you really want to replace the motor there are DC brushless servo motors and controllers around designed for sewing machines, e.g. https://www.college-sewing.co.uk/jk5...rvo-motor.html . Several people over on the ME forum have used these for machine tools.

Do you mean rigid tapping? For that you need coordinated Z feed & spindle which needs a spindle encoder and a controller to suit - more like lathe control. I do a lot of power tapping on my big manual mill which has a VFD so I can turn the speed right down. I've thread milled on the Novamill which works fine.