I've re-built 4 or 5 twinnie's in my time but my advice was more general,I own several cars, from fiat coupe's(one turbo one vis) to my Derv GT, a 2.0 GTV and I look after the wife's old honda civic and my mates K series rover and his little 1l yaris..
but Chris you seem to be a pedant and a bully, for some reason have taken what I have said and decided to run it into a personal crusade.....my intention was to give the OP some advice, and I don't think advising replacing things is wrong, the only thing you risk with that policy is spending more money...if a garage does it you actually save money in the long term as the labour is the killer.
each time I do an engine rebuild I take each component on its own merit and decide, generally when dealing with HG issues as the underlying cause is commonly something to do with the coolant system I would consider those components part of the job...it rules them out in the unlikely event that it still has problems when you put it back together.
after an experience with a BMW where I had to do the job twice because of precisely a failing water pump that gave no indication of problems when it was off the car I now just tend to replace things at specific mileage regardless.
its called preventative maintenance.
my money my choice and you can disagree all you like, the OP can choose for himself who's advice to take.
re the variator well I would consider that different as its part of the VVT and would not directly affect the HG issue, assuming this is the reason why you took the engine apart then its safe to keep the same variator as long as it is performing as it should (no diesely sound on tickover etc) ,belts are rubber and cheapish and often give no indication they are about to fail so I would change this.
its not black and white though ever, if i knew the car well and knew for sure when things had been replaced I would take a view on it, I wouldn't replace a water pump that I had only put in say 5k ago...
most people do not do their own work and don't tend to keep an eye on the lifetime of or the intervals at which engine components are replaced and we still don't know if the OP is self maintaining or going to a garage...my advice was general as it would be annoying for anyone to go get a load of work done for the car to fail 6 months later only for the garage to say '''oh you didn't ask us to replace that'' it happens all the time...
and as a rule of thumb when giving advice I go down the route that is likely to give the most positive outcome, you can try and argue all you like that things don't ''need replacing'' but it is the course of action with the higher probability of success and less issues or work to do further down the line. and if like me you are basing your advice on the assumption that a garage may well be doing it then two sets of labour is worth considering.
oh and how else exactly am I supposed to word ''if the headbolts are not stretch bolts then I agree no need to replace'' I was agreeing with you and you still feel the need to make a point of it.
and finally I am not a mechanic I am a guitar luthier, as such I frequent many guitar forums. my knowledge is considerable in this area as both a luthier and player I have worked at the very highest level and continue to do so.
however , never have I been arrogant and rude when correcting somebody else's misconceptions or advising someone who has asked a question, its always done with humility and humor.
you should try it.
you might make some friends...
the cambelt, well do you have to replace it no..but MOST would, you don't then good for you ,again its your money it all depends on the mileage on the belt when the head comes off, but I was basing my answer on the premise that it had some miles on it and as the TS cam belt interval is quite short at 36k it makes sense to change it whilst there.