could be you rad is half blocked. try flushing the system with rad flush. you get this from and good car parts retailer such as halfords. hope this has helped
very simple to replace the starter on this engine, first disconnect the battery then after jacking the car up (easier from underneath ) locate the startermotor it is just under the manifold bolted to the gearbox it is around the size of a coke can. unbolt the three bolts and disconnect the two wires going to the starter the bigger one being the live feed from the battery replace starter with new one three bolts back in ,wires connected, battery back on job done good luck