I used the Fastrax yellow insert you get with the Bullseye's in Kourse Kings at Silverstone, you can get them separately...
http://www.cmldistribution.co.uk/cml...tId=0000005876 
You can use 8th scale inserts in the Kourse Kings if you want too, just use the softest ones you can.
You can make the Bullseye's fit on normal SC wheels if you dremel the very inner mounting bead from the wheel so it glues up like an 8th scale. The outer side of the tyre will then glue to the edge of the outer mounting bead.