Is the flanged hub aluminium? If so, you could see if someone would make you one from steel (if you were able to do a basic drawing) then secure it with a couple of decent spot welds to the shaft.

I did a similar thing on a slightly larger scale for using some big wheels on wheelchair motors with quite a stubby shaft. The tubular part was a threaded connection from eBay and the circular discs I drew up in CAD, glued a print out onto a sheet of metal then cut and shaped it with a grinder, and drilled it. I then welded it to the wheelchair motor shaft and it's solid. Just need to make sure the body has a suitable cutout for removing the motor with hub attached (looks like yours does)

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