Mine won't do 83kph (52mph) downhill unless you pull the clutch and hang onto a passing truck!
Seriously. It will not exceed 66kph unless the clutch is pulled. Engine acts as a limiter.
Also tried for max speed on the bike downhill, in a tuck and drafting, 79kph. Clutch pulled of course.
As for the time to modify a stock head, bare minutes.
Well worth doing for the learning and comparison value alone.
There are 3 main modification goals, 1) squish band width, 2) squish band gap, 3) compression.
Squish band width is helpful in reducing both detonation AND reduces timing advance need.
More area helps low end torque but exceeding 50% of head area is generally not helpful.
Squishband gap is essential to be in the 0.010" to 0.040" range for effectiveness. 0.020" to 0.030" preferred.
Compression helps low and mid-speed performance and torque but increases heat, especially in the piston.
I'd believe Jag's advice on compression. With Yamaha Blaster's we set a higher limit but it is a stronger engine.
For well piped 2 strokes I've generally dropped compression and timing to let the pipe do its work.
Of course cylinder height can be adjusted with base gaskets and will adjust compression, but you still have to keep squish gap in mind. A higher cylinder typically produces a higher powerband, all other things being equal.
As for head cooling, I've had no problems with any head overheating that I could tell, but then again, our temps here seldom exceed 30c with 15c to 20c being the norm.
Steve