the bi-turbo would be very hard to make work. i don't think that the arm is long enough to fit around the side of the motobecane -- you might be able to rig it but it's not going to be worth it. There are performance motobecans mufflers you can order from france....but i've never tried them.
are you running the 30MPH version? there are three versions of each motobecane, A, B, and C. If you aren't running the top one then you can buy all the parts necessary to upgrade it to the faster model for performance.
The motobecane's can't be modified nearly as much as some other mopeds - like the Puch.