I'm all for doing a top notch repair job, but in this case, I would just use washers, then a split lock washer, and red threadlocker on the bolt. That will never come off.
Then it will be the next owner's issue.
But by the time anything in the starter motor needs replacement again, the powertrain will have other issues that will require pulling the engine, so there really would be no harm done.
If it makes you feel any better, I had to pull the engine, disassemble it and split the engine case to get to the starter clutch in my 1986 Kawasaki... That was a great week I'd like to forget...