could u convert the nc30 to use a nsr 250 smart card ignition? or is it worth getting an alarm with this function built in?

Simple answer is no!

Which function?

The idea behind the MC28 smartcard system wasn't so much security, it was each that an owner could have several smartcards, each one containing a different ECU map so the engine could be tuned for each specific application quickley and easily by selecting the apropriate smart-card upon startup. (ie track use / rain / general use)

didnt someone try to fitout a nc with that system a couple of years ago on here ?

No because it's just not possible as stated the card itself hold the fuel mapping which in turn works with the PGM unit which is basicaly the CDI, trying to use a 2 stroke CDI on a 4 stroke would be like trying to make toast in a microwave.