If your friends are mechanically inclined, you can swap out the piston in a motobecane engine in 20 minutes. It's SUPER easy, but I dont think I'd recommend a total noob to try and undertake that.
200 for a moby that only needs a piston to be installed is a steal in my area. However, you have no way to know for sure if that's the only problem. Moby's are pretty unique, and have eletrical demons - so if it has good spark, then I'd get it. If you have no way to verify that, I'd recommend taking a pass.