Hello and welcome,
first off i hope you are friends with the mechanic. you cant really take a moped to just any mechanic or you will end up paying half of what the bike cost you for 2 hours worth of work. that reason alone is why almost all of us learn to do it ourselves. if hes your friend and is familiar with small 2 stroke engines you are very lucky though!
motobecanes are beautiful mopeds. they have more oomph than most other brands due to the variating engine (that means it moves, yes moves!). however, because of this the motor is more complicated and can lead to odd troubles.
does the bike currently run? you know to mix your gas and oil at 50:1 right? that is about 3.5 ounces to the gallon. dont use cheap lawnmower stuff try to get the good shit at a scooter store. If it doesnt run and you have some extra cash ditch that carb right now. the weakest point on the moby is the crap carb they came with stock. you can purchase a 15/15 SHA Dellorto carb from www.dellortodirect.com or you can even head to seattle mopeds and they might have it and more!
The good news is you dont need to worry about a transmission or transmission fluid. so as long as your piston isnt locked, and you have spark you will absolutely be able to get it to run with the new carb. it will have more power, run better and you wont have leak issues.
is your gas tank rusty? if not get a inline fuel filter anyway (it will save u lots of trouble down the road)
I was gonna write more but my second dinner is getting cold, anyway the point is we are here to help!