Linda, this may give you a bit of a head start in tracking the boys registrations down.
If you contact Elizabeth Parkin [Fell Pony Secretary] at this address
fellponysocietysecretary@hotmail.co.uk explain your current situation with regards to the boys. If she is able to trace possibilities, she may direct you into DNA testing, there are a number of places in the US that are able to DNA for you, I will dig out the address's for you. Once those tests are through, you fax the originals but keep a copy for your records and send them to the Fell Pony Society, you could in theory ask for the ponies to have duplicate passports.
I cannot guarantee any of the above will help in obtaining duplicates, but it has got to be worth a shot :)
1) make sure you know the exact/approximate ages [it will help Elizabeth when looking through computer records]
2) where the boys have come from [current home]
3) colour and [markings if any]
4) have you had the boys ID scanned....could be worth checking to see if they have Identichips.
The late teens one, could be one of Mary Jean Goulds ponies or Wendy Ihlang if he was a bred in USA, he could be an UK import too, but if you can contact these ladies listed below, they would be able to give you a list of foals bred on or around about the time you think your chappy was born!!??
Mary Jeans website is:
http://www.laurelhighland.com/Wendy Ihlangs website is:
http://www.fellponies.com/