Re: gas tank on rear rack..
I needed to get rid of the upper tube tank for safety. I looked around and found a used Honda tank for their 3 - 5 HP engines at a lawn mower shop. I used the three mounts on its bottom to fasten it to the rear rack. I shortened the spring loaded bar so I can still hold a jacket. I used auto lacquer and clearcoat to paint it black to blend in. Later I put a glass fuel filter in line to the carb.
I looked high and low for a nice round aluminum tank like some have found, but the $12 used tank worked out OK.
Easy does it, but do it!