Lots of generic advice then but nobody around who actually knows if it is a make to kill or break to kill and how to remove the twist grip assy.
What did you expect. You own the thing, take a DVOM to the switch and figure it out. We don't know where you are, and you didn't list the model number, or year of the motor so that anyone might be able to look at a wiring diagram to help you out.
Cut the grip off, as it is, most likely, glued on. go from there with a new grip.
Best of luck