Picture shows a complete bike, not a frame. If you bought a complete bike, great, as Motobecane's from that era had a host of french threaded components, along with often a Swiss bottom bracket. Finding those parts can be somewhat costly and sometimes challenging. I've had several Motobecanes and other french bikes from that era.

Who knows whether top tube marking it original? Of more use are any frame tubing decals that specify the material used to make the bike, along with looking up date codes on components (google bicycle date codes). Realize that the only thing on that bike made by Motobecane was the frame itself. The rest was made by a host of manufacturers, who made parts for all the manufacturers.

Motobecane information on the web is somewhat limited. They went bankrupt decades ago, and have reappeared as a generic, made in China brand.