In most cases, the Gurtner carb bodies are the same. But there are some varying smaller sizes available for older mopeds that are pre- 1974. (To fit on the Motobecane Moby-X mini's as well as the Motobecane Cady with the smaller engine head.)
The intake size and the ports do make a big difference in the end speed.
I find it interesting, because on my Columbia Open Road with the Solo engine, for instance, the speed was changed via the clutch flange nut. On these when you grind off about 1/8 of an inch off the flange nut, the clutch engages "smaller" so the front belt is running smaller, giving the Columbia more speed. I've already done that and my Columbia goes around 35 stock with the flange grinded down. I've got a rear wheel in the garage with a 42 tooth sprocket. That ought to give me a little more end speed. Can't wait to install that. ;o)
On others, it's either the rear sprocket size, front sprocket, or the variator. But with the Gurtners on Motobecanes its the carb size/jet/port. Hmm..