Hi, any 1998, 1999 New beetle, or 1999.5 (new body style) Jetta/Golf ECU will work. The later cars had an immo and won't work.
If it's auto then you have to change the soft coding with vag com but it'll otherwise work. If it's a manual then it's plug and play.
The best thing to do is to remove the windshield cowling on the pass side, and take a look at the ECU part number. Tell it to any junkyard and they may be able to find the same thing.
A good auto recycler aka. the junkyard for TDIs is: L&T Enterprizes
6220 Airport Rd.
Allentown, PA 18109
610-264-9191
800-228-3895
I have no affiliation with them, they've just always had the part and known what I was talking about. If no one is there then leave a message. I've seen them for $150 shipped but that would be a low price.