An the modified One would cost more than a standard MCS to insure, and it would probably be worth less than a standard One if you wanted to sell it. Add to that that it will be missing lots of equipment the MCS has and I don't see the point, unless it's a very cheap damaged or recoverd One or for some sort of race use and the One is lighter?
Only thing you will gain is your car will still come up as a Mini One using your Reg plates when insuring. However you need two V5's one for the car and one for the engine and this might affect a few things.
We've all heard the whole "but I've done so much to my One" spiel before, strip your one to standard, flog it and buy a Cooper S. Safer, more reliable, easier to insure (legally) and cheaper (in the long run). We've also heard the sleeper argument too, you still need the bonnet scoop no matter what happens so a Mini One with a big scoop is hardly a sleeper .
If you stick Cooper S performance on a Mini One without changing everything connected to the engine you'll be forever fixing the One bits that can't cope with it.
Unless your a garage that has the resources then it's not worth the hassle.