I use an XR100 tank on my CT140, it holds 1.8 gallons. These plastic tanks can be expanded by draining all the fuel, filling with boiling water, and pumping in some air to get to the desired size. After it cools, dump the water, let it dry out, and it retains it's shape.

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Q (Quantum Mechanic)

1968 CT90/140
2007 Lifan 200
2005 Zongshen 200
1981 Honda GL500 Interstate