Is the wheel not balanced or wore funny? Bent rim? Maybe the forks are slightly tweaked in the triple trees and brace. Maybe try to get the front end up in the air and loosed the pinch bolts for the triples trees and fork brace then line everything up and tighten it all down.
Maybe when you put the races in for the steering head the cups didn't seat all the way or didn’t seat straight so it wants to pull to one side. Is there any play in the front end when you have the front off the ground and you grab the forks and push and pull them? With the front end off the ground, is it stiff to turn one way?
Other than that, the only thing I can think of is something internally in the forks. Like maybe one is missing a spacer or has a shorter spring or something. I don’t know, just throwing it out there. I'm not sure if that would cause wobble problems or not.
One last idea. If the frame has been modified, could the neck not be lined up perfectly vertically relative to the rear axle? Is the frame raked or hardtailed? Could be something with the alignment or the geometries of the head tube like rake and trail.
