Grey fox,
I looked my SEWMOR 303 over and here is what I found. The main shaft is .025"+ larger than the standard (Singer) shaft. That's why the spoked hand wheels don't fit.
However if you really wanted to make a hand crank machine out it, it would be quite easy. Just to to an auto parts store and get a break hone that will compress down under 1". Then hone the spoked wheel out to fit. There might be some end play so if the flange on the main shaft the hand wheel works against is adjustable then that would need to be adjusted. Otherwise it might need to be shimmed. This would give you the HC machine without ruining that aluminum hand wheel.
Another thing is these machines are a copy of the Singer 99 and as such they are not as smooth running as the 66s or other bigger machines. If you notched the hand wheel and some time down the road put the motor back on, that might just throw the balance off enough to make it hard to use.
Motor; If your motor is the original 1.0 Amp Model 755 motor it is worth having it rewired with the correct wire. The motor is also a 3/4 size motor so it won't protrude into the arch as much as a full sized motor.
The mounting bracket is the same as many of the full sized machines. At least the one my 303 is. So if push comes to shove, a standard motor can be used.
Joe