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If your sled was indeed a practice sled for the Yamaha team snocross racers, it makes some sense to start with an XTC instead of an SX, even though the SX was the "production racer". The long travel suspension is better for snocross, and the parts are already on the XTC to start with (shocks, radius & steering rods, trailing arms, limiter straps...), then they changed the shocks to Ohlins, likely all 4 of them.
Since blue was the team color, repaint the tunnel (yours does look repainted), radius rods, trailing arms, and hood for 1998, or replace the hood with the factory blue SX version in 1999. The blue seat could be from an SRX for 1998, or SX/SRX in 1999. A couple other items that point to 1998, or 1997, are the rear suspension wheels (aluminum with rubber tire), and the black rear bumper cover.
You could also try to find the serial number on the engine and compare with the tunnel VIN. I'm not exactly sure where it's located but I believe it is stamped into the aluminum casting area that houses the flywheel/stator.
Just keep in mind, the sled has been around for awhile and many things could have been changed over the years. It could be a look-alike racer that someone put together back then, or it could be as it was when Henry, or Titus, had it.