Fine advice Engineman!
A pile of porting might gain you a few % more power, likely no more than 5%.
Milling 0.020" off the head might see another few %. Likely no more than another 5%.
One of the biggest thing this head needs is squish area to impart some turbulence, speed up the burn, and resist detonation. While I had the valves out, I'd get a good aluminum welder to zap up some flats by the plug and on the other side.
THEN mill a few thou off to flatten everything up, do the valve seats and all the work you suggest. A cam would go well with more compression and you'd be off to the races with 20-50% more power.
Do you think?