I'd speak to some of the quick guys at your track to ask them what they run, but i would expect it to be a Proline or Losi Tyre in either M2/M3 or Silver/Red compound on Clay.
To be honest they should last a while on Clay as it's mainly the sticky soft compoud that works rather then the tread itself.
I think the ZX5 RTR's probably come with the old Kyosho H Pattern tyres which are quite hard too so if you go for something like a Proline Holeshot in M3 compound -
http://www.m-k-racing.com/product_in...oducts_id=4730 They should work fine and also come with inserts so you just need a set of rims to go with them and you'll be good to go!