The belt-fed CETME solution may be to simply copy the HK-21 sprocket type belt drive device. It is a WW-I concept. It works like a racheting 6-pointed star spool that rotates one click at a time from a long lever depressed by the bolt carrier. Pirate here on gunco just got one of these you may recall.
The MG-42 type feed (also MAG-58, M-60, French Mlle-52, Swiss M-51, and SIG 710's) requires a long receiver with a top opening. That design seems more akin to a Brit Bren or Russian DP.
Also keep in mind that the CETME and HK systems require a fluted chamber. So an 8mm Mauser caliber conversion will require a fluted chamber too.