If you have major dramas finding a vented caliper there is a seller on Ebay who sells spacers to convert non-vented calipers to vented spec for about £30. I've used the seller before and he's fine. http://cgi.ebay.co.uk/METRO-BRAKE-CALIP ... 286.c0.m14
I'd be tempted to fit some new discs on that set up too.......
Ha looks familiar. I had to buy a new set galvanized calipers on Ebay. They are offered for sale frequently, so you should have no problem finding new ones.