It keeps running hot and the thermostat is is no longer in the car what should I do

Have the radiator cleaned, back flush engine and heater core, make sure fan(s) is running, make sure water pump is pumping, I have seen pumps with rusted off impellers.

Install a lower temperature thermostat It will help you engine run cooler and prevent it from overheating.

They said "It keeps running hot and the thermostat is no longer in the car"

You likely need to install a new radiator. Corrosion or phosphate scale are causing passages to be blocked. Afterwards, refill with a 50/50 mixture of DexCool and distilled water; otherwise, buy prediluted DexCool.