Maybe the plating is worn off the cylinder wall in that spot or maybe real thin due to age , that would cause major piston failure and overheating. you did say it was a 1991. do you know if it's ever been replated ? if not thats a long time on the junk factory eletroplate. Maybe the piston and rings were too aged to handle race loads. any idea on the age of those ? I once was stuck at a bad bottleneck at a GNCC race for porbably 10 min at idle with mud packed around motor and radiators and did not overheat. I was surprised I didn't break down, Four stroke bikes were dropping like flies. The steam from boiled over bikes was like a swat team smoke grenade. Most of the two smokers made it thru.
If your crank still cycles smoothly and has no noticeable play, run it....
get a new cylinder, im sure someone here could get you one for short money, once again go back to the water pump for certainty and double check jetting once you get cylinder issue fixed.
I'm surprised no K5 experts have chimed in yet
anybody
Hope some of this info might help as I once experienced this with an older bike also. my problem ended up being worn plating and poor jetting ( the bike was an 87 K5)
Keep us posted with your results and good luck
PS - just thought of this, check your stator plate position to make sure it didnt move from normal setting maybe the screws loosend up and the timing advanced a little and overheated
