I like the way you think! Cutting up new KX450F parts to scavenge the shafts is exactly what I was considering.
I've been thinking about this with the Yamaha 125 and 250 as a durability upgrade; I break shift shafts at about 80 hours on those.
I cut splined shafts all week long here, but these shifters are so small that it is better to roll them than mill or hob them, so the factory 450F shafts may be the best way to go if they're long enough.
The welding fixture has to be splined, or at least include an edge to accurately time the splines. Of course, a fella could make a custom shaft that was fine tuned half a spline offset from stock if desired.
Making new actuating plates is actually no big deal. When I get into the engine, or get another shaft in my hands I'll punch a design into CAD and make up a small batch of the plates to try out and figure which year KX450F or YZ450F has the longest shift shaft.