[David]

Squish is probably the biggest bang for your buck. Stock squish is usually too large for reliability. On 125's we run about 0.030", but we check it everytime we make a change. If it's air cooled and/or big bore, you'll need more, if it's smaller you can go even tighter.

You could raise the exhaust port, but that moves the power band higher and may make your power under the curve smaller which could make it slower overall.

Clean up the intake track, not bigger, just cleaner. Clean up the area in the case where the intake charge enters the cylinder and match these areas.

If you can widen the cylinder intake ports, there's a little power to be gained.

You can move the power around by changing the angle of the intake port roofs. It's been a while since I've messed with them to know which way to go, if your thinking about doing this let me know and I'll look it up.

Bigger carbs can help if it came with an undersized one, but you also loose bottom and midrange by going to big, for a minimal gain on top.

Advancing the ignition timing a hair will help everywhere but on the top.

More compression helps everywhere but on top were you want less compression so the engine will reach max revs.