Upgrade old Gary Fisher MTB drive train to 10 speed Las year I purchased a old 1998 or 1999 Gary Fisher Tassajara MTB. It came with 519 Weinmann wheels 26x1.5/559. There is no brand on the rear hub, I believe they are part of those Weinmann wheels.

Will a 10 speed cassette fit? Running 8 speed cassette at the moment.
Yes. The same Hyperglide freehub body spec is used for 8 through 11 speed drivetrains on mountain bikes.