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.
Thanks viper yeah I've been studying what pictures I could find of the hk21 system I was thinking for a lower belt feed to use an upside down mg3 type with a cut down top (bottom) cover to make it fit then a new way to engage it mainly to avoid trying to make an hk star wheel type from scratch. Baisicly the two L shaped levers I thought of would have one that functions like the star sprocket but would move back to its original position and the second L shaped lever ( maybe not an actual L but angled kinda like the cetmw ejector maybe) would be pushed down by the rear of the bolt carrier, that second lever would would engage the first lever to move the feed prawl to the right (actually left since it would be upside down) advancing the belt then when the carrier was forward and clear of the second lever I mentioned a sring would pull the first small lever (and thus also the second) back to their starting position which would also move the feed prawls back the the left (right since upside down) and behind the next carteidge in line. I'm just trying to think of how to use the already availible mg3 stuff with maybe making as little stuff from stratch but it might be just as difficult getting the mg3 feed mech to feed upside down. On the other hand if some lucky guy that happens to own an hk21 would take measurements of some of the parts required for the hk21 feed system that would need to be milled I could probably get them made pretty reasonable.
By the way not trying to be condesending by explaining how the mg3 feed system basicly works, that was for explaining my mod ideas and for the benifit of others reading that might not know.
As far as my "mg 45" cetme I understand I'll need a fluted chamber but thank you for reminding me if I want a fancy barrel bravo 5 said they would sell me a fluted chambered blank (so I could do 8mm) but to keep cost down while I'm still seeing if I can even get the think to work I was thinking a cheap mg 42 barrel and then at worst cutting the flutes best I can with hand files (or even a die grinder with a long tiny carbide bur and a giude) welldog on here and I have been talking about a multi blade broach idea the might work well to cut the flute very nicly (it would have a center piece that's basicly chamber shaped to cam the cutters to folow the chamber profile) but we won't have the time to work on it for a while.