I don't see the relevance of the fuel tank?
But anyways, get autodata or find someone who has it. It has the ecu pinout diagrams that will let you wire it in. If you've cut the entire loom you'll need to make sure you wire everything back in.
A switch and push button is a peice of piss that can be done off the ignition wiring connected to the barrel, like so:
So there really is no need to touch the engine loom at all.