Eric-
What Dennis, Richard, and Steve said- I'll add caveat that I always chase ALL the engine threads, as they all seem to end up being haven for 'illegal aliens', and always mess up torque values.
Make sure that your replacement bolts are of same quality to the original head-bolts... or just re-use the originals... chances aren't very good that there's any 'strech' to be concerned with- they're not under that much load, and they're not designed-stretch fasteners like current automotives. Furthermore, the thread dimensions of many 'average' bolts just isn't up to tolerances with 'real' fasteners... I've found that the garden-variety graded bolts available at most places will mate very well with their own nuts, but the threads won't fit tapped holes very well. I'm not typically a stickler for thread precision, but having marginal thread contact patterns in comparatively hard-to-replace items just doesn't sit well with me.
As Steve noted- without retorquing (per the Kohler manual- download one from Kohlerengines.com!), the cylinder head won't transfer heat properly, and you'll get a gasket blowout. Running a thicker gasket won't help you either- it'll simply allow for more variation in compaction, meaning less-constant thermal transfer, and more susceptability to temporary thermal deformation.
And as Dennis noted- there's no Cast Iron heads... that'd be a step backward in technology, really. The aluminum head has a higher coefficient of thermal transfer... it accepts waste heat from the combustion chamber, and passes it to the atmosphere. The head also accepts heat from the spark plug, and passes it to the atmosphere. In an air-cooled engine, faster is better. Cast iron won't pass the heat as fast, and as a result, you'd get plenty of preignition problems... this is why you just won't find a cast-iron head on an air-cooled engine made after oh... 1948 or so...
Sounds like your engine took a little hurt from having poor airflow, which distorted the head a tad (and probably insufficient bolt torque) caused the gasket blowout.
Scrape out any stray carbon, chase the threads with a tap, clean the original bolts, install a new stock gasket, Put it all back together right, with the original bolts, and torque according to the book, start-run it for the prescribed time, then re-torque, then live long and prosper! V
(Message edited by dkamp on September 15, 2004)