Onstep certianly improves the CGEM. At its best I was getting around 1arcsec rms tracking error in PHD2. Its now half that with the onstep conversion. At a focal length of 1m (with an 8" F5 newt) I was getting eggy stars and these are gone with the onstep conversion. If I was you I would first put it back together, adjst the backlash, and see what its like.
For the conversion, you bag the celestron controller board and motor. Keep them if/when you want to sell it. You will replace those with some sort of onstep and stepper motors.
- have the onstep control board inside the mount (like the celestron controller board). If you want to do this then use miniPCBver1 as it fits nicely in the mount. In my instructions there's a 3D print for a faceplate to mount the miniPCB
- use any onstep board and have it external. Print a faceplate with some connectors to the stepper motor inside
- read the wiki several times. Then have a look at my instructtions (there's a link to them somewhere in this thread) as they are specific to the CGEM.
- The stepper motors have to be mounted inside the mount. I have 3D print files for the brackets.
- the big things is replacing the cogs on the worm shaft with pulleys for the belt drive from the stepper motors. This is awkward to say the least. But given I am not very mechanically minded and could do it - anyone can do it.
If you go ahead with it, have fun and ask any questions you have