You can definitely build on the current frame, but do keep in mind that if you'd like yours to be a "true" cafe racer, the rear seat loop should be present and that would have to be welded on. Of course that isn't too difficult, you could source a rear seat loop from Aliexpress (ready-made available) or have it made locally from steel tubing similar to the one used in the CG frame. The tricky part would be getting a seat to fit onto that new setup since I assume you wouldn't want to use the stock (fat) CG 125 seat, but instead have a thinner cafe racer style seat made with the rear hump and the works. You could work out a new bracket system to secure the seat to the frame properly! Shouldn't be too difficult..