You appear to have covered all the bases, so I would suggest you have it checked by another mechanic.

There's no doubt in my mind that you are up for the service charge; you asked them to service it and they did. You are correct that your car isn’t worth a lot of money, but it’s worth almost nothing with a cracked head. You could perhaps talk to the dealer about getting a second hand engine from a wrecker and having that fitted. It might be a cheaper option to get the car going.

There's no reason to give you continuing warranty, the company has seemingly repaired your car when it has given trouble and on that basis is likely to continue to do so. If a part regularly gives trouble, as you say the inlet air temperature sensor does, you have good justification for Great Wall to make repairs as and when needed, at their cost. Remember also that you have recourse through the consumer affairs laws that protect you rights under our laws.