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Regarding the jack shaft and dual chain drive, a couple of thoughts.
Chain could be the same size as the Cub uses, not the size that suits the rear wheel for a 600sports bike. However you're accelerating more mass so going smaller than the Cub chain size could be an issue.
Second thought, a single (long) toothed belt or chain with an idler on the back of the tension side (on the jack shaft) might be a simpler and lighter option than a pair of sprockets and 2 chains. However, the sprocket pair gives you more gearing options and allows easy chain alignment for the wide rear wheel so there's a trade off.
To get a second set of gear ratios, there have been dual chain 2 speed "gearboxes" made. Scott had one on a bike in the 1920s and people have made them since then. The jack shaft allows this option more easily because you could have 2 chains to that and a single chain to the back wheel. The dual chains should be chains, ideally, as they are narrower and take smaller diameter sprockets than toothed belts and have less drag.
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