I used this tank, which came with my first Staton friction drive kit w/1.6hp Subaru engine. I didn't like how it looked, but it served its purpose. The gas did leak from its rubber gasket one day and sloshed onto my butt.
I've used goped tanks with engines having no saddle tanks. I've also used Happy Time fuel tanks alone, and in series with the engine tanks.
I like the 4-liter HT tank the best. If you fill at the pump, be sure not to put the oil in first and pump slowly. The oil will not mix properly. When you open the petcock you'll have a VERY heavy oil/fuel mix feeding into the carb and stalling the engine. After you flip the bik upside-down to clear the carb, you'll end up with a smoky engine, then a VERY lean fuel mixture. Of course, there'd be no way to calculate your actual mix left in the tank, so you'd have to discard it and mix a fresh batch.
Ask me how I know all this.