They look like they are made to fit larger (12mm?) hex wheels, like the DF03 or touring car size hex. I'm not certain, but that's what they appear to be.
I broke the kit hex (on my 501X though) and there are no other options that I know of apart from the proper Tamiya items that fit straight on. You can fit other similar small 'losi style' hex's - such as the HB d4, Yokomo BX, AE B44, Losi XX4 etc etc - but you will need to drill out the hole to 5mm I think in order to fit it over the axle.
I use HB D4 hex's on one car, B44 hexes on another and kit hexes on the third.
Jimmy Storey - www.oople.com