The 4500 is a touch weaker shaft and bearing wise, mainly the input shaft. On clutches the stock unit in the 5 speed seems to be better. How the clutch holds up will depend a lot on your driving style.
I don't know who else makes one, but www.genosgarage.com has them, dunno if it's a B&D or not. Put up a post in the vendors section or give Mark Craig a call and see if he has one.
BTW, they are nice, had one on the goat for about two years now :rockwoot: