If the original spindle drive still works, then I'd use that.
Failing that, I'd either buy a KBIC board, or a Sprint drive, as they'll give more stable speed control than a cheap chinese controller.
One thing to be aware off, is these types of SCR drives often have non-isolated inputs which sit around half of their supply voltage, so need isolated speed control.


Spindle encoder wise, the original setup on these has a single slotted disc, and a multi slotted disc.
In terms of what you'd need, will depend on what motion controlled you use.

You need to be aware that not all motion controllers support turn/lathes and threading.
I'm not sure what UCNC's current status is regarding lathe functionality, or what they'd require in terms of spindle feedback.

The ballscrew setup on these is very basic, and ultimately rely on the stepper motors for endfloat control.
I personally wouldn't bother with a closed loop setup on one of these, as they won't improve accuracy.
Most accuracy problems I had with mine was due to the noodly Myford ML10 base machine flexing.