Hope you have sorted out your fan selection and fitting riri.. Whatever fan you have decided to use, perhaps best to check the manufacturers advice on the selection of an appropriate fuse.
I note that Richard advised that the fan in his post runs at 12.4 amps and if that fan might be selected a 30 amp fuse would perhaps risk creating a similar situation to the failure of fan and wiring you have already experienced..!
As gomog suggested it seems likely that the relay seems likely to have carried more current (amps) than it was designed to and may have damaged it`s internal contacts, thus a replacement relay suited to the replacement fan`s current and a modern relay with internal diode protection can help protect the relay`s internal switch contacts from burning in time.. using a relay that can carry more current than the fan requires is unlikely to cause a problem, thus a 30 amp relay should be fine, though once again the fan manufacturers advice is best followed..
Also perhaps worth thinking about is the possibility of wiring up a fan over-ride switch, which can be used to over-ride the radiator mounted temperature (otter) switch that switches the fan`s relay on and off..
The over-ride switch can act as a back up should the otter switch fail.
Hope you get it sorted out and avoided any engine damage riri.
