Kiwi,
The thicker head gasket is to lower the compression a tad. This may be something to consider,
http://www.kxriders.com/forums/index.php/topic,3158.0.html The carb spacer is avail from Jerry "JFab" I got one from him, Less than Moose and excellent quality. ( If you have the gumption, you can make one yourself at very little cost.)
Just like lengthening the intake tract on automotive engines, this will move torque lower in rev range at some top end sacrifice.
You also mentioned re-gearing, again, gaining bottem end giving up top end.
The guys here have worked out some head modifications that improved performance, (squish angle and width). This will help out across the whole rev range w/out any downside, at least to my knowledge.
Another consideration "if your budget permits" is a PWK 38mm Airstryker carb. This will give you more of the "snap" in the low and mid throttle,however... sacrificing top end. The 38mm airstryker also uses the same type needles and jets your present 39.5 pwk uses.
I think several other guys will agree just going through the old girl and bringing her back up to snuff, (Reeds,seals,rings etc.) will give you a good baseline to start with and may surprise you how much the bike has in it stock.
Tuck\o/