The Thermostat
The Thermostat Housing and connector, the pipeclamps & o rings.
Flush the system (new g12)
Check all your hoses (look for pink stuff around joins), change/tighten any pipeclamps, particulalry around the oil cooler

Thats it.... if its taking ages to heat up its usually the thermostat stuck open opened. You can also check and see if your temp gauge is working correctly...