I think that the rad fitment is the last option.
There could be a number of things to explain this.
First thing to do is check the fluid level.
Then if ok leave the car idling after a hard drive and check that the temp returns to normal and the fan kicks in and out.
If the fan works and the temp is still high then maybe the thrmostat is not functioning correctly.
To check this, remove and allow to cool ( and therefore close) then fill a clear container with boiling water and it should open fully.
If this is ok check for fluid loss over a period of a couple of weeks- park car on a level ground or a place where you alway park and mark the level on the bottle when cold, then simply monitor overe two weeks - you shouldnt really lose any.
If so you may have a head gasket issue, that is a different chapter though, do the basics first.
