A few years ago someone gave me a way too long Riptide motor. He got it from some auction for a couple bucks. It ran fine but unless I wanted to use it standing up it was too tall. I removed the control head, then took a piece of 1/2" CPVC and ran it down over the wires where the cut was going to be. I then cut the shaft using an abrasive wheel. The CPVC tube protected the wires.
I didn't even bother to cut the wires, I just doubled up the extra and stuck in down into the tube and reassembled it. I cut out about 18" or so total.
The shaft is like a giant fishing rod, just resin and fiber, cut it just like you would any fiberglass or composite material.
I saved the cut off piece of shaft, filled it with spray foam, and put a grip on one end and its a floating fish billy club for the big boat. Sort of like a heavy duty little bat.
The motor lives on my crabbing boat now as back up and a means to move around short distances without starting up the gas engine in the shallows.