Hi,
the cast iron cylinder is a straight swap for the alloy one, and if you buy a used cylinder it will come with the right head studs anyway, I'm not sure that you need to change the cylinder base studs, though. I built a spare engine for my 90 staring with a knackered bottom end from a 90 Sport, end fitted a 90 Deluxe top end onto it. It seemed to fit OK. One extra thing you will need to change, though, is the Exhaust Clamp nut - the alloy cylinder has an internal thread, whilst the cast-iron one has an external thread. Also, the Exhaust pipes are different part numbers - I'm not sure what the difference is, though. It may be a different bend angle or maybe slight difference in length. I guess that's a case of try it and see.
Graham