choppa do a search mate, there is an absolute wealth of information here on it.
If you havnt already bought a bike your best bet by far is to hang on until one of the regulars sells one on here; you'll be covered on all bases and probably be able to get an ivanised one too. They don't come up often, but they do come up, patience is the key. Your only sure way of checking whether an exup valve is still in situ is having a gander inside, but as you'll find from searching, that aint always an easy thing with the age these bikes are now.