I had the timing belt changed at the dealership and the heads and valves redone at another shop. The car has been overheating. Changed the thermostat, the fans and had the radiator cleaned, yet it is still overheating.

have the fault codes read first
if the transmission oil has not been changed , that would be a good move after reading the fault codes

it could be a broken cam shaft

does it have gas

as the engine is based on a gm unit you will need to take off the head,sorry.....

True, the engines are low tolerance engines which means that there is no room for the pistons and valves in the cylinder walls at the same time. When the pistons are TDC and the valves are open, they will hit causing damage to the valves and pistons.
Be more cost efficient to replace the whole engine, rather then just the valve train redone. Chances are the the pistons will have some type of damage and it will not just be one piston that will need to be replaced.
Thank you for using Fixya and sorry for the bad news

OK!... did you do a cylinder 'leak down test'?.. .if not, do it! If any 'unacceptable' leakage, have the head REDONE! NOW!.. you must BE SURE the valve timing is accurate!... Go to www.hmaservice.com and register( Vehicle by VIN). Afterward, you have access to shop manuals, service bulletins, wiring diagrams, etc. about your vehicle.

Sounds like coolant ports in the engine block is gunked up. Did you maybe notice the coolant ports when you changed the head gaskets? And, or, change the radiator cap,if you haven't already done so, I've seen them heat up over that too.

If you want to make a more educated decision and not rely on others, you will need to buy the shop manual for the car. It will show it all. How to diagnose, to check, what tools are needed, torque specs, and set by step directions. It does sound like it overheated, If your friend saw that the belt tentioner was defective and that it needs a serp. belt, ask him to show you what he sees. After that repair you can then test the cooling system and see if it is back to normal. Thermastats are said to be changed every year but ive seen then go 5 years without problem. Do find the sorce of the overheating, if it is the thermostat then change it but if it is the waterpump then concider having the timing belt changed at the same time. It will be more cost efective and the belt is suppose to be changed after 80k miles. Good luck.

That Can Only Be Determined After Replacing the Timing Belt Or Removing the Head, It has Been MY Experience that About 20% DO Have Valve Damage. This is Aftet MY 30 Years As A Mech/Tech. I Normaly Advise Customer Of the Chances And R&R Timing Belt and as I Say Toyota Is Better than Honda About NOT Tapping Valves when jumping Time.

check for a vacuum leak