Summary

I purchased a knock-off exercise bike from amazon and the resistance device failed within 1 month of use. It's essentially a threaded rod that moves a magnetic strip closer/further away from the flywheel. The OEM one was made of cheap steel so the threads stripped. To replace it, I purchased a 1/4-16 ACME threaded steel rod and designed a Peloton Knob to thread on top. Enjoy!

Ended up doing a boolean cut of this part from McMaster-Carr to get the internal threads. It may be too tight to assemble if using a hard plastic.
https://www.mcmaster.com/93420A881/