Can I just replace the attached thermal sensor?
Apple MacBook Pro "Core 2 Duo" 2.6 17"
Is it possible to replace the thermal sensor that's attached to the heat sink? Or do I have to replace the entire heat sink?
Lets try this first. If you have your system put back together can you download this fan & sensor monitoring app: Temperature Gauge Pro. There are others I just like this one. The free trial version should be enough for now. Post a screenshot of what it shows for us to see.
Your problem maybe something else (unless your sure you killed the sensor). In any case this will tell you if you did.
If you did kill the sensor you'll really need to replace the heat sink as it's mounted quite well (epoxied). You will also need to clean off the old thermal paste and refresh the thermal paste with a goo brand.