I've contacted Daryl from Mossy Creek Outdoors (very helpful - thanks NIW and RK for the suggestion) and he has the parts I need - just sorting out postage cost for these and some hydraulic steering parts as well (that'll be another post).
It's a good thing, as I had a closer look at the large bevel gear and the shaft this afternoon, and in metric units they are both harder than a cat's head. The file skated across them and there's no way I'm drilling it with a standard drill bit, and not a chance of tapping to use grub screws as scotch keys. Maybe I could sharpen a carbide masonry drill bit, but.....
Since I've hopefully got parts on the way soon, I'll replace the leaking seal and put it back together with new dowel pins to take out some of the play and get back to using it.