I was just joking about the Neu motor when I noticed that it fitted the space you had - its more at home in a heavyweight .

Running two brushless motors on one shaft is fine, its just the ESCs that have to be separate for the reasons Mario mentioned.

The disk design sounds fine, you could probably thin the spokes a bit as they are quite short, or even go down to three spokes to get more weight on the rim. If you can spin that up to a high RPM, it will be very effective.

You can replace the shaft in an outrunner motor fairly easily. There is usually a circlip on the shaft at the bottom of the motor; popping this off with a small screwdriver lets you pull the entire endbell off the motor. The shaft is held in the endbell with two tiny grub screws. Loosen them with an allen key and then gently tap out the shaft with a punch and a hammer ( I usually end up using a long screw as a punch). Buy a length of silver steel the same diameter as the original shaft , cut it to length and do the reverse of the disassembly procedure. You are better of without the circlip groove in the new shaft, its a weak point where the shaft snaps.

If you are mounting the motor on an aluminium bulkhead, there is a great opportunity to add a support bearing to the shaft. Here is a photo of a motor mount I made with an extra support bearing:



The motor mounting holes limit the size of the bearing you can fit, but I discovered that a needle roller bearing is perfect for the job; it takes high side loads and has a small diameter. If that sounds useful, we can work out exactly what bearing to get.