You need "Fallshaw" 400 x 8 KNO rim and tyre, with 2 PWB10 black adapters, to take 2 of 6004 rubber seal (20mm), high speed bearings.
All times 3.
I get mine from,
http://staging.easyrollmh.com.au/wheels-castors/pneumatics/plastic-pneumatics/ The very bottom of the page for rim/tyre, across for bush PWB10 and with the second line down of the bearings 107mm x 20mm, for 20mm axle shaft.
Don't worry about their speed rating they are really well balanced and have been pushed to over 100kmh.
Hope this helps Andrew. Yell out if you get stuck.