Here is an article written by Jesse Stephenson from the Moped army, comparing the 50V and the Motobecane 7 models:
The 7 uses the AV7 engine, but the catch is that it uses only the case, crank, piston, cylinder, clutch (dimoby 1 speed only). That's it. Everything else is different. The stator is mostly the same but the place where you attach the wires for are different. They are not marked and are both the same size, unlike on the 40/50, where they are marked 6v/12v. The wire for the coil is also shorter so if you swap the stator over to a 40/50 it wont’ reach. Operation wise its the same. The engine is not mounted by the head but by the carrier that mounts to the case, so the head is different. and the motor mounts are different. The mounts also are in a different orientation so the motor will not accept a variator even though the bottom is mounted on the spring. The rubber motor mounts are the same as the right side on a 40/50 so you can swap those over if you need to. The intake, carb, exhaust, wheels, frame, tank, pulley, pedal shaft, all different and will not swap well to other mobys.
And the weird thing is that even though they pretty much only came in 1 color and 3 speed ranges, being the cheapest bike Motobecane offered, they made a huge amount of custom parts for the damn thing. For performance, even stock, you can get the thing to fly. They have 2 carbs, the AR1-10 and AR1-12, a 10 and 12mm carb fromGurtner. Which is very similar to the AR2-13 which is stock on AV10 engines, but not quite the same. They also make a 14mm AR1 that has no markings on it that is also slightly different. I can’t find what it came on, I just found a box of carb bodies at Handy bikes.
They had 3 different intakes, a 10mm, a 13mm and a 15mm. They had 2 exhausts, one is open at the header, and one has a weird pipe in it that cuts off the flow. The bikes also only weigh in at about 80 lbs wet so the fast moby cylinder was never used on this model, even in the 30mph version. The 20 and 25 mph version also had a 39 tooth rear sprocket, while the 30mph had a 35. They also made a high comp head and a low comp head.
Also the rear wheel is cup and cone bearings, with a different hub and will not fit on a 50 frame., nor can you use the sprockets on a 50 wheel, the inner diameter is different. The wheels also have 28 this gage spokes and smaller brake hubs than 40/50 so they tend to bend really bad when you hit big things in the road, although they stop really well. The pulley setup is also just for this bike, with a completely different outer diameter, freewheel engagement system, and shim setup. The shaft that it rides on is only slightly shorter than those on 40/50 and will clear but the sprocket on the pedal side grinds on the side panel tab. As far as I know the only front sprocket they came with was an 11, but its also not compatible with the front sprockets they use on 40/50s.
The petcock threads the same but points in a different way, so you can use the replacement 40/50 style but the fuel line then points to the back of the bike, which looks dumb and keeps you from using the sweet fuel line guide.
So for super stock performance, find the 15mm intake, a 15mm del fits right onto it with the plastic shim, the open header pipe, swap a fast 50v cylinder onto it, high comp head, 11:35 gearing, and time it 1.7-1.8mm btdc. They move pretty well that way, and you don’t even have to do any custom port work. And of course, make sure everything is working fine on it before you try to make it faster, saves lots of headaches. Man, I do love my 7’s. Good luck with it.