Did a couple of things today - made a mount for the capacitor and wired it in. Also made a new earth wire between the relay and engine block, as @nomadpete pointed out, the old one was a bit short when you factor in engine vibration.
As per the picture below, while my "new" fork is big enough for the new donuts, it's also a fair bit wider than the previous one - axle and central shaft shown. The old shaft seems to be 5/16" threaded both ends with locknuts.
The new one will have to have a non-threaded length of 220mm plus enough for the nuts either end, so probably 250mm all up. Probably the best way to use the existing axle will be to buy a couple of pieces of round aluminium extrusion (an offcut would do, they would only be around 40mm each), drill the 5/16" hole through the centre, then at one end of each, drill out to 16mm to sleeve it over the axle. Thanks @Kyle Communications for the idea.
Question is - where do you buy high tensile rod, zinc plated? Assuming I'd have to tap the ends myself - I only have a metric tap set so could probably do 8mm and bore out the axle if necessary. Any ideas most welcome!