Finally after a long dig through paper and digital files I found, what I believe, is the last surviving picture of a mini bike my dad built me in 1969.
I believe he used a pedal scooter type frame, welded on a back bone to adapt rank and seat. He raced sidecars, so he couldn't resist to bolt on a 3rd wheel. The engine was a 50cc moped engine, believe me, for a 8 year old this was mighty fast. I had fun, however, eventually ended up flying into a corn field and the bike caught fire. That was a great show for the neighborhood.