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

unless you are on lpg :)
just detonation then... it loves lean (to a degree)

i reckon timing first, then mixtures (from issues i had with su's on petrol)
but is the head gasket on right way too? i know a few members have made that mistake, causing a restriction in the flow of coolant and oil.

running rich is not just bad for economy
but will build carbon deposits (pinging)
Breakdown the oil faster (dirty oil)
Possible glazed bores over time (faster rebuild)

I'd go with the timing. If you raise the compression, the burn will be quicker/more complete - which means you need less advance (a little less initial and total, so you can get away with just dialling in a couple of degrees less initial) - if you kept the timing where it was, it'll be doing a little bit more 'negative work' as it burns and expands before the piston has passed over TDC, so a little less advance gets this 'push' happening when it should. 2-3 degrees should do it (if it took more than 4 degrees I'd be surprised). It's also worth noting that it can have too much advance but it still won't be audibly pinging at lower rpm.

I'd hold off altering the mixtures for the time being. Going richer only lowers the temps if you go richer than stoich. This would only be called for if it's really bad at full throttle. When you are running part throttle, you aren't getting anywhere near the same effective compression/volumetric efficiency, so going rich would be costing fuel, but not actually a major benefit, as you (shouldn't) be near detonation from mixture alone. It gets even more interesting, because as you go leaner, it does run hotter, up to around stoich a/f ratios, then when you lean even further from there, believe it or not, chamber temps actually drop off again. Believe it or not you can actually run cooler at part throttle by going leaner than stoich.

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Running lean is my guess. Have you richened the mixtures any? Increasing the compression causes the air to get hotter & burn easier, so if you haven't given it more fuel it will run hotter. Try the next main get size up & see what difference it makes.

you might be runnin it a bit lean try richin it up if it turns out the temp sender is alright

Had the same issue after changing heads, at the end of the day it was the new Temp sensor I installed.

After sitting with the sensor, a kettle, bunch of resistors, multimeter and a thermometer; I figured out were the needle needs to be on the gauge with the new sensor.

It hard to say without knowing what all you did or changed on the motor. I would say check temp sensor, timing make sure its not running to much advance, and make sure you got the right heat rage plugs in there. The list goes on but it would be nice if you can tell us what you changed. Good luck

Did you change temperature sensor on the head? If so, put the old one back on.

Have A12 engine. Upped compression to 10:1, now runs 3/4 the way up the temp gauge.
The radiator core tubes look clean, and has new antifreeze. My choices are 1) have radiator cleaned, and 2) put A/C water pump and clutch fan on it.
Any ideas??