Does anyone have free plans for a ultralight?
Your best bet is a Sandlin Goat, with free plans for download and quite a few built.  It's a soaring glider but some have added motors, although that eats into the useful load.  A motor would have to be a 2 stroke to have adequate power for the weight, (you want about 25 hp) and it would be best to use an off the shelf paramotor engine.  I would just purchase an old used paramotor cage/motor to get the motor/reduction drive/prop and rebuild the motor.  A goat itself is still going to cost you at least 5k to build from scratch, probably at least half that in the cost of fabric covering process, before the motor.  Then of course you have to engineer a way to attach the motor with adequate structure that can handle several Gs, doesn't screw up the weight and balance, etc etc etc.

Aviation, even at that level, ain't cheap.

But before all that, you have to get training to avoid killing yourself, so save up a couple grand first and go to an ultralight flying school for lessons.