Does the 998 still use the half rings as valve keepers? It's quite common for them to break and you're supposed to replace them whenever you change a shim. But, even if the break it's usually not a problem as it can't fall out. Their only about a quid each, I think.
Nelly will know more...
Oh, and to refresh Paul's memory:
MBP valve collets JHP do them in this country. I've looked into it seriously for my race engine, but (1) they are quite expensive, and (2) you need to machine the shims to fit on the 4 valve heads. So reshimming becomes a problem unless you also buy their special shim kit. I passed.
[Edited on 17-3-2004 by Felix]