An aircon water pump will do nothing as it does not pump any more coolant than the original. The difference is in the thermo fan mounting.

Bring your variables under control by .............
Using the previous temp sender unit, or a new one.
Using the previous thermostat, or testing the one you have, or fit a new one.
Pull radiator hoses & test radiator, or have it cleaned and/or repaired by a professional if needed.

Sometimes pulling the head can disturb crud deposits in the block resulting in rusty looking crap flowing into the radiator & blocking it after re-assembly. In a stock 1200 only about six tubes are actually visible through the filler neck, leaving a lot that aren't

Re-assemble & test drive. Unless you are flogging the bejeesus out of it, any mixture error will be slight & should not noticeably affect the temperature