please help-had an engine change

Hi every1

I don’t know much about cars I need to stress that first. Ok I have 86 MA70 non-turbo supra and I just replaced engine with the second hand one that has done only 70 000 km. my mechanic says it’s a top notch engine, the best he could find. But on the first day after 30 min of driving it started to overheat. I noticed that the water in the radiator is red. My mechanic said its just rust and that I need to wash the radiator and the engine. Well I washed it 4 time now and its still overheating and water/coolant is still red. I cannot return the engine so what should I do.
I have the same problem before this engine change only now I have a sticker, few stains and scratches and I’m 2000$ poorer. And yes my brand new radiator is now probably stuffed up too.

Can any1 please advise what I should do?

Thanks

These cars are actually pretty easy to work on. I would start with the thermstat. It would take you less than 15 minutes to swap out. How high is the temp gauge going when it overheats? Does the car cut itself off? Does it seem to have the power it should have? Does it crank right up when you start it?