The tornado was 13khz I believe.Trying to get coils for it will be near impossible now as the machines are no longer made.Sometimes you can find them secondhand but this is becoming more difficult.They were made by a man called Vic Fiveash in a small workshop at the bottom of his garden in England.They were originally made to search the foreshore on the river Thames in London were there is a lot of iron contamination, where the machines ability to pick through the iron was highly regarded.No two machines are exactly the same as most customers had the machines made to their own specifications and that is why two tornados can look considerably different.Vic's machines are still highly sought after,but sadly he died a while back and nobody can repair them if they go wrong.
If you want another coil it might be worth contacting Kevin at staffordshire metal detectors(England) and ask him to look out for one.....he ships worldwide and his sevice is second to none.He is probably your best bet to get coils for these machines.Might be worth sending him an email.
All the best,neil.