Hi F5
Don't know about a seal kit/repair kit but here in Europe you can buy the complete tap and filter for 1996 to 2009 model for less than 18 euros.
GasGas part number is BE25610031
https://www.motocrosscenter.com/shop...0485-104x.html
The 2018 model fuel tap for just under 11 euros, GasGas part number BE76020GG-CLJ-1
https://www.motocrosscenter.com/shop...gg-clj-1.html#
I've never had a fuel tap apart, is it just O rings inside, if so could they be measured up to source replacements?
All the best, Dave.