I'm in the process of planning the conversion of my recently acquired Novamill, and came across this during my research - https://www.summet.com/blog/2016/01/...-speed-control. It shows how to tap into the existing spindle drive isolation circuit with your own PWM signal, thus saving any new circuits or power supplies. In my case I'm just replacing the top controller board, and replacing it with a modern motion controller - but keeping the lower board and the stepper drivers - so this should work for me. Sounds like you may be replacing more of the electronics, so it may not be appropriate. But hopefully this is useful info for others reading the thread.

Cheers
Graeme