I was contemplating to buy this, however the dual axis drives alone sell for $199. I can't justify at this stage to spend $400 to replace my motorized CG-4. I'm open to people persuading me though.

Celestron screwed everything when they started counting their GEMs. Actually, the CG-5 was the first with the Celestron German designation. Why 5? Probably to distinguish it from the Vixen GP it had been selling. Lets see they had the EQ-1 they sold on entry level scopes, the Polaris, then the EQ-2 sold as the G-5 with the C5, I guess the latter was the CG-2? So the AstroView/EQ-3 became the CG-4 and then we have the CG-5. Above the 20# CG-5 there was the G11, then the Celestron International 700 (CI-700). Once Synta started selling the CG-5 to other vendors they couldn't call it the CG-5, so it was sold as the EQ-4 or EQ-5.

The SVP is an outlier. It's from Jinghua, which makes the EXOS GEMS, Why Orion wanted different castings for the Dec and RA housing isn't clear. Perhaps, Synta had an exclusive license from Vixen to copy the GP.

It's fairly easy the distinguish the EQ-3/CG-4 from the EQ-4/5 or CG-5 by looking at the latitude hinge. The latter is noticeably heavier in its construction.

I've been playing with building a simple RA drive for the GP clones. Here's what I've come up with using 3D printer components.

Pretty much everything sourced from eBay, although you can find similar items on Alibaba or Bangood a little cheaper.

NEMA 17-400 step motors with 20 tooth pulley $9 each.

NEMA 17 motor brackets $2.50

40 tooth pulleys (GT2) $7. These were 5mm; I had to bore them to 6mm to fit the worm axles.

Toothed belts $1.

A handbox and Arduino based driver circuit for driving RA at sidereal with 8X east and west corrections costs another $20-30 depending on the driver selected. OnStep Go2 starts about $50.

For smoothing out the stepper's pulses µstepping is used. Rates of µstepping start at 800/revolution at double steps to interpolation of 256 µsteps (102,400) steps/ revolution. For the CG-5 with it's 10 minute worm period, direct drive would use 1.33 steps/second at double steps to 170.67 steps/second at 256. The 2:1 belt reduction doubles that. Thus 32 µsteps per step (42.67 steps/second) is considered an acceptable rate for AP. Stepper drivers that support that rate start at a buck.