You can't change the gearing without replacing absolutley everything from the motor backwards, as far as I know.
The NC-5 version is leagues better but shouldn't be £7 more expensive, you are looking at the wrong dealer. I paid £25 (I think...) for my black NC-5 one, swapped the motor for an NC-2 (due to club rules) and won the evening with it. Sorted.
Anyway, why do you care? I thought you were going to swap it for 'the best NC-2 shaped motor'... In which case choose the best livery, which coincedentaly is the black NC-5 one...
Lotus