Hi and thank you for your question.
For starters, it is absolutely not true that there are any sensors on your bike that would prevent it from starting because of overheating.
I am concerned that whatever sealer your poured into your system is going to do more harm than good. My concern stems from the fact that motorcycle cooling systems have very small passages compared to car systems, and any type of coagulant sealer would clog those passages nullifying any cooling capacity. Exactly what brand of head gasket sealer did you use?
The other thing that bothers me about that is head gaskets blow not because the gasket failed, but because the head warped and no longer supports the gasket. The high pressure created by combustion then damages the gasket, but because it is not firmly squeezed between the head and cylinder any longer.
What you need to check is start out by checking for spark. Since your bike is carbureted, as opposed to fuel injection, the only ignition controlling sensors have to do with whether the bike is in gear with the side stand down or side stand up, in gear with the clutch pulled in.
I suggest pulling out all 4 spark plugs and looking closely at them. Right off the bat you want all 4 of them to look alike. If one is darker or lighter than the other 3, or 2 are light and 2 are dark, we have issues. They should be about cardboard box brown. If they are all really dark, and especially if they are sooty black, replace them all, then re-test for spark. If they aren't sooty black or dripping in oil black, put them back in the spark plug caps and ground the spark plug steel casing the the cylinder head. Keep the tips of the spark plugs away from the open spark plug holes just in case there is gas in the cylinders. If you have spark on all 4 cylinders, then you need to do a compression test. Do that with all 4 spark plugs OUT and the throttle wide open. You do want to leave the spark plugs in the caps and grounded so you don't damage your ignition by not providing a path to ground for the electricity.
Lastly, you will need to check for fuel getting to the spark plugs. First I would drain each of the 4 float bowls, into a suitable container, and check for water in the gas. Water looks like an air bubble running around in the bottom of the gas. If you see water in the gas, you will need to drain your tank and refill it with fresh gas. It is quite possible you got bad gas from a gas station, through no fault of your own. It happens, several times a year.
Those are the 3 things required to make an internal combustion engine run. Spark, Compression and fuel. We just need to test to see which one is missing and repair it from there.