discs are interchangeable between twin wheel and single wheel, and twin wheel had either solid disc or vented as an option, but single wheel never had vented! Should be easy enough to find vented discs, I founds loads of different ones about 12 months ago when I bought some for mine. If you do fit solid discs, then you really should split the caliper, remove the spacer between the two halves, and bolt back together. Try this crowd, I bought some caliper rebuild kits off them, they list the vented discs:
http://brakeparts.co.uk/#page=parts&pag ... .0+OHC+VANConrad