Well, I didn't like the rim as it was (see photos), but since it looks like there's no OEM replacement, I opted to make a little splint from aluminium at the back of the rim, with holes tapped 3/16 BSW. I've screwed in dome-head screws through the plastic rim into the splint. You can see in one of the photos the template I drew to mark out the aluminium splint, for cutting it out with a junior hacksaw and some chain drilling.

The only thing is, I have used brass screws just to get it done, but I will have to buy zinc-plated steel screws (I don't have any in my odd screw jars); this is because brass and aluminium are not good metals to be in contact - there is a strong risk of galvanic corrosion. But at least the job's done and once I've got the steel screws, I can press the rim in and get the mower going again.

[Linked Image]
[Linked Image]
[Linked Image]