What type of riding are you doing?
Where do you want improvement in powerband?
Pre-gen is already optimised for top-end power; it makes max HP pretty close to redline. All that's left is higher-lift cams with adjustable gears. One-tooth advance is too large and rough of adjustment, you'll lose all gains for all mods combined and more.
What is done in many cases like this is to narrow down the ports to improve mid-range torque. Latex is poured into port and pulled out to model its shape, then extra metal is welded in to narrow ports. Then blended in with grinding and polishing.
In end, you'll have something similar to new-gen head.
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