Is the original nipple damaged or shot out, or the threads in the nipple seat damaged? If not, I would recommend retaining the original nipple for firing blanks.

The hard part is matching the threads on a repro nipple to the original threads in the English nipple seat. Most of the repro nipples are threaded to fit repro guns made in Italy, and the threads are different.

But here is one dealer - there are many - here is a page with a repro Enfield nipple on it. From the description, this should ("might") fit original English threads.
http://www.dixiegunworks.com/default.php?cPath=22_93_463_464_388

Hope this helps,

Geoff