chances are it is the radiator however if your rig has been neglected a good part of its life then it certainly could be a combination of things. if you plan on keeping the brat, then at some point you will have to do something about the entire cooling system. So too should you address the the engine tune, etc. as these things become older , the cheap quick fixes over the past many years catch up and it may now be the time to renew/restore the existing system.