VFD retrofit Hobbymat BFE65

[birchy]

...to variable DC speed control...

I assume you meant AC control? If so, the mechanics are probably the hardest thing to do...in particular getting a motor small enough to fit the machine without giving up too many horses.

Regarding motors, the basics are that a 4-pole will run at approx 1425RPM at 50hz and a 2-pole will run at around 2850RPM at the same frequency. The big decision is whether to choose the slower 4-pole (for more torque at low speeds) or the higher top speed of the 2-pole but with reduced torque at low speeds. You can use stepped pulleys or gears to overcome this but I just didn't have the room for such niceties on my machine.