I can try and figure it out next week, I have an empty Buell motor and a empty evo Sportster motor for mock ups, and can see if its possible to figure out..
It never occured to me to put a BSA top end on a sportster, the later EVO sporties and even more so the Buells are about as advanced as possible and quite an engineering feat compared to the old Ironhead sportsters.
However, One project Id like to play with is trying a Sportster top end, both Evo/Buell (All alloy) and Iron head or possible Shovel or pan/knuckle on top of a BSA motor. I have seen pictures on line of a BSA preunit single with a HD ironhead or Shovel top end, but have not heard how it was done. IE: Rod/stroke-piston, bore, cams etc. I have a copy of the photo here somewhere,,
But if someone knows HOW or the details of fitting the HD top end on a BSA single Id love to hear about it.