HELP: 86' 3.8L V6 poor performance, not warming up

1986 Mustang convertible, engine code 3, only 59,000 miles. At 31K mi overheated and blew the gasket due to plugged OEM cat.
After my friend, who runs a legit restoration business, fixed it, I thought that engine lost its crisp performance. At the time, I was too busy to look for the reason. Last 6 years car spent winters in the shed (summer car). Last year, my woman lost the serpentine belt. Took it to some place where they installed new belt, and added an ORANGE anti freeze. It gelled up on the way back. Engine overheated, but did not blow the gasket.I flushed it. This year, runs fine, but lacks power. I have more time, so I started an investigation. First, checked timing with strobe light and set to specs. It turned to worse. Then, I checked engine temperature. It peaks at 152 degrees after a 10 mile drive. I changed a thermostat, plugs, wlres,distributor cap-no improvement. Temp is still low. Air pump sounds like sewing machine, probably bad. ?DID MY CAT RUST SOLID, AGAIN? WHY ENGINE DOES NOT WARM UP? ANY SUGGESTIONS? THANX!