Thanks MarkT 

I'll try to get a picture with a good angle to see how much space is available when the chain is on drive sprocket.  When I feel around, there is about a finger width space  (about a1/2 inch to 5/8 inch) between the shaft end and the drive sprocket

My plan, so far, is to JB Weld this pipe end cap to the casting where it's broke without getting JB Weld on the shaft end.

 

 

Sounds like a good plan 

If that's the end of the selector fork shaft that's visible, it seems as though the circlip locating the shaft is probably missing, there should be a 5-6mm gap at the end of the shaft to fit the rubber coated blanking plug?

Ideally the motor needs to be stripped, the missing clip replaced, and everything else checked at the same time?

What you have suggested might well work fine, but would be better if you can get an aluminium plug made, with a shoulder on the outside, to stop it going in too far, and provide a little more area to bond to.