(Last post didn’t appear to upload - having another go!)
I bought a Rotax 14T drive gear - also available in 12 to 15T, there is an 11T but not sure of the bore dia, all the others are 3/4 inch so I believe.
Next was a bush to sleeve the gear from 3/4 to 5/8 for the Torque Converter and a 1/4 x 3/16 length of key steel plus a thrust washer to get the spacing.
The drive gear needs the threaded portion machining off and I did reduce the thickness a little to get in the thickness range of the gear supplied with the TC, when added to the thrust washer.
Then its a matter of adding a key slot*, this is hard good quality steel, but managed to grind a half circle using a rotary diamond burr file, finished off the key with a triangle file - it take time but is achievable!
Finally bush and washer needs slotting and cutting to length then all can be assembled.
All looks to be a nice snug fit.
* If you have access to a 3/16 broach this would be easier, given it can cope with the hard steel.
I have now ground back the fins on the TC in the pic
