Defender 200tdi refresh

Hi, I have a 200tdi converted defender which I've owned for 15 years. I've put a new chassis on it about 6 years ago and am now thinking about the engine. I would like to use it to tow my enclosed car trailer and so want to try to optimize the power of the engine. What is the best way to do this? Rework the head? or a complete refresh of the whole engine?

Thanks