A
As Arctic2009 mentioned, none of these mods are going to be cheap, but here are some of the common things guys are going to save rotational mass.
Port track, as mentioned.
BDX Lightweight recoil
BDX lightweight brake rotor
BDX lightweight track shaft
BDX - I believe their gears are lighter when you do the lightweight diamond drive kit?
Fastrax P85 primary clutch conversion
After that I think you have to look inside the engine for additional savings, and if you think anything above is expensive (per pound of savings), going internal is going to make it seem cheap.
Yes bdx gears are lighter, you can buy just their gears and put them in your gearcase. Instead of having to buy their whole case, and not have a speedometer anymore. well with the 05-06 case anyway..