From your posting I assume you have narrowed down the overheating issue to the fan not cutting in.
Have you located the heat sensor controlling the fan?
If so you should be able to simply short circuit the connector ( U-shaped wire on the end of the cable connector. The fan should then cut in. If it doesn't - there may be a problem with the relay, a fuse or other electrical issue. Then follow a process of elimination.
Let us know how you get on and give as much info as possible.
All the best.