My bke does this now....The frame (or engine) would usually be considered earth/negative.
So quite often something (e.g. the starter motor) will have the (starter motor) casing as ground/neg. The starter motor casing touches the frame which is ground so that's your negative.. and it only then needs the single red positive wire.
So to get a ground all you need to do is make sure whatever it is is touching the frame/engine/etc.
What is it you're trying to do?