Update: I thought my Incline did not gear low enough for the, um, inclines around here, so I bought another shaft drive unit, with lower gearing (1.77x rather than 2.07x - gives about the same lowest gear with the 7-speed hub as the higher-geared drive would with an 8-speed hub). I got it from Dynamic Bikes, they said it'll fit the Incline and it did. Replacing the unit is not hard, they sent me an illustrated PDF explaining the procedure clearly. The hardest part is removing the cranks, since I don't have a crank pulling tool had to take it to a bike shop for that. Removing the rear wheel, BTW, turns out to be quite easy, for those of you worrying about "what if I get a flat".

One thing I've now learnt is that there are thin spacer washers on the rear axle between the hub and the shaft drive cover plate, and one needs to adjust the number of washers (from 1 to 6, typically 3) for smoothest operation - too few or too many and it runs rough. That may explain why my old drive unit ran just a bit rough?

So now I have the old, standard-gearing ("street") drive-shaft unit left over. (440 mm length, axle-to-axle - the narrow shaft part between the housings is 303 mm long.) If anybody wants to buy it let me know. As far as I can tell (I bought it second-hand) this bike has seen very little use, so there shouldn't be much wear at all on the drive. I rode it about 10 miles, with no problems.