So, the 5 speed box is in. Still a bit to do.
Having snapped the adjuster, I need a new decompressor cable. More tricky, the rear left side bracket of the fuel tank needs welding back on (a single tack weld blob was all that was on it from the factory). I have a mate who apparently knows how to this without blowing us all up. I'll hopefully get that done towards the end of next week (or buy a new fuel tank and find a new mate if it goes badly wrong...)

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The new "English made" clutch cable didn't fit as expected. My Minda handlebar levers don't have adjusters, just an 8mm aperture. The upper, crimped on end ferrule on the old outer cable for the 4 speed is a fairly substantial, stepped item of 10mm/8mm (same as the front brake cable) but the new one is plain and only 6mm in diameter so it had nothing to "bite" against in the 8mm aperture. I was going to send it back but in a sudden flash of inspiration this morning I realised that I could make a slotted adapter from a 12mm long, M8 "Allen" screw with a 10mm head, by drilling through it's length on my lathe and then cutting a longitudinal slot with my angle grinder to allow it to go over the inner cable. The plain ferrule on the cable sits in the top of the screw where the Allen wrench would normally go (the hexagon shaped hole needed slightly drilling out to 6.5mm before it would go in). The new ferrule adapter has the added advantage of effectively lengthening the outer cable, leaving some spare adjustment at the other end, which I understand is a known issue with some cables.
"Any road up", as they say in these parts, the clutch is now all working very nicely, in fact it seems to operate more positively then the old setup, which was a little spongey.
Built like a gun... could go BANG!