The three factors that seem to be the most important starting out are suspension, geometry improvements, and brakes. Unless all these are reasonably good and set up well, no weight savings in the world will overcome the lack of confidence problems the first three will saddle you with.
Most racing cb750s are leaning toward 30 to 45 mm offset trees and raising the rear with a factory length swingarm by using longer rear shocks(or add on extensions for normal length shocks). Rob (Voxonda) favors 45mm trees with a longer swing arm and longer shocks on his endurance racers.
Racetech springs and emulators up front, and good shocks(Works being the most popular)of the ideal length in the rear seems to be the go to setup. Brakes are highly dependent on the class(s) you run in.
It might be worth considering focusing on geometry, suspension, and brakes initially. Starting with even a dead stock motor definitely has it's advantages. Wider power band helps with factory gearing, and the more modest power could deter you from pushing too hard while you learn to be more comfortable at the track.
I'd much rather ride a good handling modest powered bike than a peaky high powered bike with so so handling. And frankly, the latter can be dangerous, especially for someone new to racing.
Adding engine power is easy(not necessary cheap though
)
George