Wiring diagrams are in the manuals. There are only parts diagrams for the 2000 model, but assuming you have the kokusan 2k3 (big flywheel, wires exit the case at 2 o'clock), the wiring diagrams from later models would be fine.
Gas Gas Manuals
For basic lights and a horn, there is something to be said for making up your own lighting loom. MattR did a write up:
here
Not sure how much Gas Gas would charge for a wiring loom. If you are in the UK, pro racing does lighting kits for enduro bikes:
proracing