Welcome to the Forum, Some years are the same the newer they are. I have never mixed parts due to warranty & different Gear wearing issues, always use new parts here at the shop. It is best to remove and check out the Needed possible repair & buy a new part when it comes to Transmission Parts. The XB Buells are somewhat more powerfull when it comes to TorQue than a sporster. The Trannys really take a beating from some riders, this is why I really preach about using 75/90 synthetic Amsoil for best protection. Redline works really well for the Buell/Sportster style Tranny.
If you have only lost 3rd gear then most likely it's the Shifter Drum or the [Shifter Cam Assembly] as it's now called,Check out the Outer Primary parts and look for damage to the Shifter Forks first. Post up when you can. [Buellister Parts] used parts in good condition, Look him up,& May be able to help you with that. Hope it's an Easy fix.*Jimi