Having done an engine swap in a tin top I can report all you need to do is send off the v5c with the new engine number to the DVLA. Nothing else is needed.
For MOT the emissions are always based on the oldest part - chassis or engine. BMW can provide a report on the engines age if required. You can get it here
Customer.Information@bmwfin.com I had to provide the donor chassis number and the engine number and the sent me a birth sheet in the post.
Personally I would aim for a motronic engine, something from the late 80's.
Can't comment on the cab but on my sportster I have no roll bar on the front and the E30 325 touring one on the back. A stiffer rear end would lead to more of an oversteer tendency. I've not had the Sportster on it's handling limits so I've not found it a problem. The mechanical grip has been fine for all normal road driving. (a little power oversteer at low speed is easy to coax out of it with a dab of throttle and less precise clutch movement
)