tl;dr: The chance of a working AC on a Hungarian train is low and it's getting lower with every passing day.
You are in luck information wise because an exceptionally good writeup on the state of affairs was just published two weeks ago: http://www.regionalbahn.hu/2022/07/magyar-vasut-2022.html run it through Google Translate, I guess.
For air conditioning it mentions
In recent weeks, international trains have started to run out of cars, and the technical condition of the vehicles that are still running is dancing on a knife's edge, especially in terms of air conditioning.
it is a sad proof of climate change that the air conditioners of the older passenger cars specified in the nineties cannot withstand the current summer temperature and regularly break down due to constantly running at peak performance.
Expect massive delays, crowding and overall miserable circumstances.
If you don't want to do the train, there are a few relatively good options.
- Car share. The main car share in Hungary is oszkar. But finding a car which goes as far as Brașov will require a lot of luck. Try Târgu Mureș and maybe a bus but see next point.
- Fly to Bucharest with Tarom or Târgu Mureș with Wizz Air and then find your way from there. This is another question for someone more familiar with current Romanian situation than me. I do not know, for example, how is the AC on the train from Bucharest to Brasov.
The best option is to wait until November when the airport is scheduled to open.