I’m pretty sure parts are not interchangeable from the tuber style Buells and XB models.
If memory serves me right, there are some internal transmission parts from 03 to 09 that are not interchangeable with XB models.
From 03-09 there were many changes to the Buell motors, transmissions and electronics.
I can’t remember exactly, but I do recall that when I rebuilt my transmission I had to order parts that were specific to the 05 to 07 bikes. I have no idea why. I just recall reading that the parts I ordered were for those years. But it’s been a long time since I rebuilt the transmission and when I did I wasn’t looking at the interchangeability of parts. I just wanted the right parts.
I would highly suggest just replacing what’s worn out, and using parts that are specifically made for your model year.
What Cooter mentioned, a total custom route, might be the way you’ll have to go. With the right tools and knowledge anything is possible.
I’m sure there will be comments about cost. The truth is any modification done to a motorcycle isn’t about the worth of the bike but what the bike is worth to you. You know what I mean?
Let us know what you wind up doing and updates on the progress.
It’s exciting to me to hear of someone so passionate about their bike that they repair a transmission, as opposed to simply replacing the bike.
Good luck