definitely need an encoder to do proper threading. Normally I'd say on the spindle not the motor but from the looks of it you have a toothed belt drive so slipping is not likely - and if it does it will be obvious!
The CUI's AMT programmable encoders (models 102 and 103) have been working for me, but might be tricky to install on your setup - needs a small shaft to go over. Readily available from lots of sources (I use Digikey).
Trying to reuse the slotted disk will probably be more trouble than its worth.