I would think if any IGH is going to do it the SX3 would be the one.
Seems the Nexus which I initially thought some what fragile is holding up under 3/3.5hp Briggs 4stroke use. These engines make a good bit of torque at lower rpms where the build is ridden most and they survive. I doubt a tightly wound 2stroke would make much more torque. The trick is in the shifting. For your hiperf engine feathering the clutch for a smooth blend in would be important. Its a sudden loading that is rough on the hub.
Heck if you can recoil start the CG run a Nexus, they are cheap enough. I have bought complete wheels in the $75 range.
Heres an interesting idea. Years back my daughter left me with an engineless VW beetle. I just happened to have a 25hp Briggs V-twin. I installed a large torque converter on the Briggs, made a mount to marry it to the VW trans axle then hooked everything up. It was cool. Had no formal clutch. You just put it in gear, and took off. When it was time let off and shift. The neat thing was each gear had a huge range that was automatically adjusted by the TC. I could get 60mph on the flats which is very similar to the domestic(german) 28hp VW. Neat thing it would automatically "downshift" while in fourth to easily climb hills. Really very little shifting was necessary and it would take off quite well in second. The local Briggs rep wanted to feature it in the quarterly magazine and I wrote an article only to have it squelched by upper management because of possible liability issues. Oh well.
Moral of this long story is imagine a 79 or a 212 thru a TC to the bottom bracket driving and IGH. You could run a very tall ratio that the engine usually couldn't handle and the TC would make it possible on take off and on through the range. Also each gear would have a large range of speed. I could see some high terminal speed with this set up.
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