IIRC, motobecane calls for 25:1 for break-in, and 32:1 for general use. With a modern synthetic oil, you should be fine running 40:1, or even a little higher.
If you are too lean on the oil, the initial effect is that the bike makes more power (less drag from scraping oil off of the cylinder walls, squeezing it out of bearings, etc) up until the point where it runs dry and begins to seize.
And yes, motobecane made 20mph models (which are actually rated for 24mph). If it's really pristine, take a look at the sticker on the top of the airbox. It should have the speed rating on it - 20, 25 or 30.
But the loudness is concerning me. Is it exhaust noise, or mechanical noise from the engine? The motobecane exhaust is very effective (quiet) and the motors are also mechanically quiet. If yours is clattering from the top end, it's time to check compression