Sorry to answer a question with another, but how do you know that your highest speed has failed?
I'm helping an owner (1999 too) change his completely failed fan on Wednesday, and although I've got some rebuilt fans advised him to get one of these:
https://www.ebay.co.uk/itm/172536315...d0AAOSwfVpYpx0
Justifying the purchase by comparing the low price vs the work involved changing the fan. Even though this will be my 9th or 10th fitting, I still estimate about 5 hrs work.
Keep well