Life's simple Tom, if the pricks at the bikeshop won't help you, don't buy from them.
How the hell are you supposed to know the part #, you're not a bike shop. Luckily, you know someone who can help.
The part number is 19217-657-003. If I need genuine parts I've been using Sydney City Motorcycles at Campbelltown, they are one of the bike shops who actually give a shit about the old bikes. If you want a hand to do the water pump I'm happy to help. If you pay for the parts I'll stick them in for you, I might even give you some shroud and swingarm stickers for the old girl to finish it off.