Why is your stock tank annoying you?
I like to use the Happy Time fuel tank as the main reservoir or auxiliary tank for my non-China Girl engines. My favorite is the four-liter tank, which usually mounts on the top tube. However, on my latest build, the expansion pipe extends over the top tube, so I moved my HT tank onto the rear rack.
When the HT tank wouldn't fit, I've used 1.0 liter and 1.5 liter goped tanks on my builds. Don't particularly care for them, since I always lose the cap and the large fuel grommet tends to leak. My engines don't get great mpg, and I hate topping off goped tanks on a daily basis. With the 4-liter tank, I ride to the gas station and fill up every 8 days.