well going back to the days that I owned a mini cooper I had a k+n filter, straight through single box 2 inch zorst, high lift scatter cam, twin 1.5 inch su charbs, lightened and balanced flywheel and that was RR,d at 98 bhp
not bad for a 75 bhp 1275cc engine, tick over was quite lumpy but it rev'd to 8k no probs
I would say that the cam and the flywheel made by far the most difference
sadly it needed a stronger gearbox but hey it was good fun to drive