I bought a used car in the state of Minnesota for $1900 back in August and now I found out at the auto body that car needs frame repair. my rights?

It depends. If you bought the car "as is," the seller has no responsibility. If the seller made no representations or claims about the shape of the frame, even directly, he has no responsibility. Only if the seller made claims that the frame or the car as a whole did not need repairs, would you possibly have a legal claim against him--and then only if the damage is something that the seller would have known or reasonably should have known.