Thanks to everyone who's given advice on this - greatly appreciated.
From what I've gathered my engine (ME63444HE / 218225) started out as a Mk2 2000 and I have no idea what's been done to it other than it starts first time, pulls well, sounds great and there's no horrible noises or smoke. It does start to feel a bit strained around 70 mph but I've sort of assumed it's because it's non-overdrive - but maybe I'm wrong.
My current thinking is to replace the head with a recondition one that's been converted to unleaded as well as replacing the cam to a 308778 version. Unless the pistons need replacing I'll keep them, domed or otherwise. I'm hoping that these changes would bring the engine closer to the spec of the original Mk2 Vitesse. So, if I go ahead with this would a modified GT6/Vitesse head fit straight onto the 2000 block or would I need to carry out further work?
Robin