FYI
From what I remember about my old Ariel, this water tank seems to be about the right shape, if not size.
http://cgi.ebay.com/ebaymotors/BRASS-OR-BRONZE-WATER-TANK-FROM-PEARSON-SAILBOAT_W0QQitemZ250169391959QQihZ015QQcategoryZ31281QQssPageNameZWDVWQQrdZ1QQcmdZViewItem