1999 Chevy Cavalier wrong antifreeze fluid in my 99 Chevy c

Engine problem
1999 Chevy Cavalier 4 cyl Wheel Drive Type unknown Automatic 100000 miles

ok, so I'm stuck between a rock and a hard spot here. I put the wrong antifreeze in my 99 cavalier, and ran it for possibly months without any problems. Now, after replacing the water temp sender and the thermastat, I have figured out that I put the wrong fluid in, and I can't get 2 min. Up the road before it overheats on me. So I was planning on getting a flush, but the mechanic told me that I could have to replace my whole system, and it could cost about 2400.00! For me, my options are to fix it myself, or buy another car. I am opting for choice (a) considering I can not afford another car. So my question is, if a simple flush does not work, what parts do I need to buy? I know probably the radiator, and the water pump, but is there anything else to consider?

Thanks,

Tom