The only significant difference between 99-02 1g, 2003 2g, and 2004+ 2g is the countershaft. EVERYTHING ELSE IN THE TRANSMISSION IS THE SAME! (at least based on part numbers on partzilla). Gear shifting parts are slightly different, but that's easy to eyeball https://www.mad8v.com/blogs/blahg/shift-star-detent-arm-spring-compatibility
1g 99-02 and 2003 use SHAFT, COUNTER (NT:13) 24120-19F00
2004+ uses SHAFT, COUNTER (NT:13) 24120-27G00
the unfortunate thing about this... is that 2003 engine cases are different from 2004+ JUST BECAUSE OF THIS
CRANKCASE SET | MODEL K3 11301-20860
CRANKCASE SET | MODEL K4/K5/K6/K7 11301-20883
I still need to work on figuring out how to tell the countershafts and cases apart though since visually they're the same