Clutch centre is definately different, the splines are a different configuration. Same with the gearbox sprocket. You can keep the same gearing as they're still 1:1 in top gear but you might find you can go up a tooth because there isn't the huge gap in ratios below top gear. My understanding is the extra gear is effectively where 3 1/2 would be on the 4-speed box so you don't have to balance full speed cruising in top with the annoyance of having the thrash the engine up hills in 3rd at 45mph.
I think they have a different gear lever and kickstart too. Maybe a different clutch cable, which one would depend on the type of levers you have.