any of my 76 bore A12, stock A14 and 15 through the years would take off without a hitch from 2nd gear and 4.11 rear. Using a 4.8 will make it even easier.
That clutch replacement is on the back of the engine not the gearbox, the gearbox bolts up to this with the input shaft but a stock A series clutch could be used with the deep male shaped spline to suit bmw input spline much like the T50 conversions.
spline from BMW to use on A series clutch
http://i165.photobucket.com/albums/u6 ... ods%20Apr-09/IMG_1486.jpgThe only issue is designing a bellhousing and this is interesting as the BMW input shaft and engine unitary bellhousing is very shallow, therefore only a shallow bellhousing would need to be made which will save on billet.
you can get a better idea on this link
http://www.bmwlt.com/forums/k1200lt/4 ... lacement-help-needed.htmlalso k1600 or 1.6 180hp engine with 129lbs of torque also uses the same kind of gearbox
thats alot of power and torque is astronomical more like a 2000cc modern engine.
It would look something like this

or in steel using 2 water/laser jets steel pieces and steel tubing

The starter could be incorporated as original not in the box like bmw uses
and original bmw starter and side mount angle grinded off :)
bmw box has mounts at the bottom so no probs about mounts but some mounts
on the custom bellhousing would also help
eg
