I'd recommend giving the guys at Mike's XS a call. They primarily work with the XS650 but I'm thinking since they focus on the XS then they might be able to help with the 400 too. I'm thinking the generator brushes might be universal on XS' models, and if not the 650 ones could probably be modified to work. The forks (apparently you were unaware they're called "forks" and not front shocks
) might be the same. And give Ron Ayers a quick check, if they still made the XS400 in 1990 I'm guessing the major components wouldn't have changed.
http://www.mikesxs.net/