I've never had a gearbox wear out like that, so no experience of it - or the replacement part featured in the link. Doesn't mean it never happens, just not to me (yet !).
The metal gearbox conversion we are working on is really to satisfy my own craving for a reliable ,full power 24v conversion. The item featured in the link wouldn't help with that as it would be just as weak.
It does look like a good way of rebuilding a worn gearbox though, but some idea of price would be useful to compare with the cost of buying the part from Adam Harper (assuming it's still available ?).