Alternative fuel tank idea.
I gone to this website. repairclinic.com and typed in fuel tanks, just to see what they may have. With some creative ideas of making a mounting bracket (I'm thinking something behind/under the seat). I think some of these tanks, might be good candidates for motorized bikes. I found about 9 of them that I think might work. Most of these are under 12" in length and range from about $30.-$90. in price.
Part numbers that I liked:
2426284, 1658981,1611028,1659100,1659012,1729103,1651365,16 44160,1640390 and 1729102.