If using a worksman or other similar trike that does not have a top tube the options are limited.
There are some mopeds that use a 45 degree mounted tank the Honda PA50 is one.
Also using a gokart gastank mounted at the rear. But you would need a diaphram carb or fuelpump for either and the tanks would need to be modded to use duel fuel lines for the diaphram carb.
For a trike due to the extra weight and extra drag by it being bigger a 2-1 friction drive would be an good option