Just because you can doesn’t mean you should. It’s not a bad thought actually. A 20 inch wheel tricycle would be great for pre-teen child who didn’t have the best balance. It could also potentially look very cool.
The problem here is the execution. The step in back is a good idea but the bare metal is an invitation to disaster. It also appears that the step is a structural element. That’s bad as well. Each half of the back will act as a torque arm and will put lots of pressure on the joins to the original frame. To prevent this you’d need to add a couple metal arms to a point more forward on the frame, being careful not to interfere with the pedals. That’s actually the reason so many tricycles have double steps in the back.
Then there’s the always problem of a straight axle. It will not turn well without a differential.
Finally, it appears that rear end is being held up by wooden blocks. That does not inspire confidence.
With just under 4 days to go nobody has bitten at the $50 opening bid.