You can get taperlock sprockets with various chain pitch, shaft diameter etc such as
these for example . You need the other half of the taperlock too (the bush to fit the shaft).
(Edit - looking at the ranges it might well be that a bush for a 50mm shaft means the smallest sprocket might be too many teeth for what you're after, but something like 18/26 looks possibly viable. Another edit - looks like those are 530 sprockets, thickness 9.5mm or so, but you get the idea of what's available)
I know it makes sprocket changes a little more time consuming, but you probably won't need to change the rear one that often. Industrial chain isn't rated for auto/bike use, but the sprockets should be OK. Don't be tempted to use industrial chain, use bike spec chain.
Not quite the same league but I once fitted a Honda C70 into a kart for a customer who wanted a "quiet" kart for his lad so it didn't annoy the neighbours too much (a big driveway!). I used one of these taperlock sprockets on the rear axle for the same gearing reasons as you describe.
As an aside, on the chain tensioning business I'd definitely use a fixed idler sprocket, and fit it on the "slack" run, not the tight run. You can probably find sprockets with centre bearings off the shelf, just need to fabricate an arm and adjuster (in fact on the same site there's
these idlers )