So I had a test fitting of the motor. Everything else went pretty nice, except my attempt in finishing the adapter plate contruction in place turned out to be a little challenging. In the end I also decided that I'll drop the flywheel and the clutch altogether and go for a direct coupling. It'll save weight and I noticed that the electronically driven tachometer didn't want to work anyway without the ICE in place. So after I had got the wheels spinning I took everything apart again.
As you can see it really is better to be lucky than to be good. One of the bolt holes in the drive shaft support just fits a hole in the motor. I'm not sure it's quite enough to hold the support in place and if it will still match after I've taken the flywheel and clutch out, which may alter the motor position.