The mower project has been put aside for a few weeks. It seems I stripped the adaptor that connects the motor to the transmission. It was lightweight aluminum and I wasn’t sure it would hold up well, and it didn’t. I started looking at shaft adaptors again. I considered these originally but I was mostly worried about damaging the motor and transmission shaft. I found this shaft adaptor on Amazon that looked interesting. Turns out with a small amount of filing it was an all but perfect fit. It is also lightweight aluminum but it is a larger device that is certainly an improvement from the bolts and drilled pulley I had rigged up.
i was loathe to take everything apart again, but it wasn’t so bad. I was able to get the adaptor fitted to both shaft ends and even gave it a brief test. Seems solid and had less vibration. I will put the seat and fenders back on and try it out in a proper test run in the next few days.