Originally Posted by Enerarevir
I had a **** of a time because, even though it was under 50 cc and had pedals.
I think you answered your own question there. I'm not a lawyer, nor am I versed in Puerto Rican law (heck, I can hardly remember how to spell it), but most legal entities consider these contraptions to be the same as mopeds and hold them to those standards.
To be sure, though, you should contact what passes for a Department of Motor Vehicles down there and ask them.