The old bronze castings are going to be around for a long time to come, the worm drive and dc motor not so long and to replace them it's almost the cost of the whole unit. An industrial 063 size worm drive can be obtained much cheaper.
6mm alloy plate

12mm alloy plate


The windless shaft key way need to be extended 30mm and also machined to 8mm. With the combined plate thickness of 18mm the shaft length is perfect. An 050 size worm drive could easily be adapted.

I wish I lived near you.. I'd turn up at your shed with broken things and beer..