I've got the same problem over here too. The shaft length (6.0 hp Tec) is significantly shorter than the drive hub for the TAV. I can just bolt it up, an it would probably work, but a short extension on the end of the c/shaft to fill the void would sure be nice.
Plus, I saw just such an item the other day, but on a small 5/8" shaft.... actually a small keyed extension that was held in place by the end bolt on the crankshaft.
If it would have been a 3/4" shaft, I would have bought the whole motor just to get the extension.
mardyn