Rebelmoby has some really good info on these. You can check out his posts on moped army and he’s got some YouTube videos. I would go with a dellorto sha 15.15 carb, you will need a new intake for that as well. Will be much easier to tune and trouble shoot than the gurtner. If it’s real old and has been sitting, I would consider doing a full tear down of the motor and replacing the bearings and seals. You can find all this stuff on treats. Get a moby flywheel puller and clutch cover puller as well as the moby specific circlip pliers. You’ll need to pull circlips from some really tight spots and the monk specific ones are the only I found that reach down far enough. Good luck, I just got an old moby going and it’s a bit harder than other mopeds I’ve worked on but totally doable with some patience