There is a dogleg or a bulge in the intake port on the heads. The pushrod hole goes very close to the intake port, and thus there is a bulge. Port this down, but don't go too far because you don't want to break through. Grind it down so the port is 1.10-1.15" wide (use a set of dial capilers to measure). Also on the oppsite side of the port there is a eddy or a dip. Polish this smooth.
What kind of springs you get is determined by your cam.
Adding bigger valves adds more power, but IMHO it's just not worth it because if you dump a bunch of money into stock heads you could have probably bought a set of used aftermarket heads.
Little more info for you....
http://www.cmc.net/~xero/Mousesporting.html
-Josh, aka the tireburner