Hi 'fred,
factory is 17/34 except that my '84 Heritage came with 17/36. I swapped the front to 18 then back to 17 after finding that the new Heavy-Duty O-ring chain wouldn't clear. Ended up running 17/33 solo then back to 17/(& carefully saved)36 with the sidecar.
With it's hardtail/girder-fork frame your bike presumably weighs less than stock so the proposed 17/31 sprockets may well be OK.
Buy a new good quality chain with those new sprockets or you'll be sorry you didn't. And a ScottOiler too.
And even living in the woods you still have to start every ride from a standstill, eh?
About your axle running out of thread, what I'd suggest you do is this:-
When you pull the back wheel to swap it's sprocket, thread the castle-nut onto the axle until it's notches are fully aligned with the cotter-pin holes.
Measure the distance between the inner face of the castle nut and the inner face of the axle's end flange (the round head with the hole through it)
That's your stack-up distance.
The distance all those axle plates and wheel spacers and chain-tensioners and whatever have to add up to so the axle can be fully tightened down like it should be.
Full width axle plates and correctly machined spacers is best. A big stack of 20mm plain washers will work too. So long as the stack-up distance is right.
