the CG125 frame is one where engine is stressed member, meaning its part of the frame bracing, the kawi frame you have is not, its a cradle frame.
if you can shoehorn the engine to sit between in the cradle then you need to make a heavy steel mount bracket for the rear bolts, to accomplish this you need to keep the rear swing arm pivot exactly where it is - ergo measure it correctly - not just futta and feeta but correctly make a jig that can hold a centering piece over the axle hole. that way you can place it back on exact spec after you have welded in the new bracket for your engine.
If your cradle does not allow it then you need to modify the frame to be like the international market Honda H100S/H125S frame. you already know the H100 frame is same as CG but with missing front leg, the international models had the leg.
c'mon you are a custom builder - its easy as cake.