Sorry for seemingly abandoning this thread: I had camera issues & an update without pictures was pointless. Finally managed to borrow a camera & take some quick shots this grey morning (apologies, it looked much better when I took pics with my dslr in glorious sunshine last weekend).
I ended up buying a 1990 Koga Miya Terra Runner & an extremely beat-up 1991 Terra Runner Carbolite for parts.
It's triple-butted, spline reinforced FM-1 chromoly steel with beautiful lugwork:
Full Deore DX, Suntour xc comp chainset, Nitto stem, handlebar, & bar-ends, Sakae Strong seatpost, San Marco saddle, Wolber AT 20 rims.
All components are in excellent condition, & mostly original, except for the LH shifter-brake levers (some late 90s crap v-brake sti) & the suntour xc comp chainset.
I immediately replaced the LH levers with those from the Carbolite, so now it's mostly matched again, although the LH shifter is DX push-pull and the RH is XT push-push. The rapidfire shifters feel rock solid, but I still think I'll replace them with thumbies before the big trip. Anyone have a pair for sale?
According to Mombat, the 42/32/20 Suntour xc comp chainset only came out in 1992, so it's not original. The granny ring is also 56 mm BCD, so I'll replace it with the 94/58 mm STX cranks from the Carbolite.
U-brakes get a lot of bad press, but mounted on the seat-stays I can't see how they'd be any worse than cantis. Does it really need replacing?
I originally planned on getting new wheels, but the Wolber AT 20 rims look super tough & barely worn. Koga Miyata hand build all their wheels, I don't think I could build a stronger wheel myself.
So far I've ridden it 100 km on roads, 50 km off-road. It's a great ride, very forgiving on gravel and bumps. The true test is how it handles fully loaded, but I haven't decided on panniers vs trailer yet. I think I'll rent a bob yak next weekend & see how it climbs and descends.