Spindle Encoder for 7 x 14 mini lathe

Does anyone have any experience adding a spindle encoder to the Harbor Freight/Vevor/C3 model of 7 x 14 mini lathe?

I have two goals:
1) know the position and of the spindle for operations like threading
2) - next step: control the spindle speed from LinuxCNC.

But for now I want to focus on the spindle encoder,
I thought about replacing the motor with a servo motor with built in encoder, but I am not sure if that is feasible,  Although I suspect it is if I added a VFD or something, but I could be totally off base on that one.

Any advice will be greatly appreciated.  I can send pictures of the spindle if that helps, and it does seem to have a 4 'contact' hall effect sensor attached to the end of the spindle already.  I assume that is how the readout on the lathe knows what the spindle speed is but I haven't dug into that yet.

Thanks,
Kent

The simplest option would be to belt-drive a commercial encoder from the spindle with a toothed belt in a 1:1 ratio.

It is also fairly straighforward to make an encoder, using a drilled plate and three opto sensors. Or two if you don't plan on using rigid tapping. Or, in fact, just one if using missing-tooth indexing.

On the same theme as that last option, you might want to look at a car crank sensor, as these have space for a nice big through-bore:

If possible, I replace the BLDC motor with a servo motor and it works perfectly.