Pistons won't be an issue. These engines will tolerate cams of up to 290 degrees without modded pistons as the are not of domed type.
Cams will work, but they will need a specific map to make them work. Also, any real performance cam for these engines will not utilise the variator as valve to piston contact would be an issue. Anyone who claims to have 'racing cams' with the variator is either just a liar or plain out of their depth.
If you were to go for cams, you would need to remove the head. Polish the ports, and skim the head. Buy a new gasket set-£80 (if memory serves- cheap ones are simply not worth it), cams are probably best part of £450- remember new cam followers. A specific ECU chip will cost another £200- if you can find an off the shelf one, and then why on earth would you use the original exhaust manifold.
I spent over £1000 on my 75 Twinspark and did all the work myself (apart from renewing the valve guides). I now have a realistic 165-170BHP engine (others would no doubt quote about 20BHP higher), which develops its urge about 1000rpm higher up. It has changed the character of the car (I planned this), but the extra urge is not really that apparent. What now happens is that the car now has the same urge when it is going 10MPH faster!
If you want to feel greater acceleration, get NOS or a turbo, but don't expect a night and day change with cams and things.
I don't mean to sound so negative, especially as I love the difference
, but I think that you are looking for bangs per buck, and I don't think this is a route which will give you the value for money you're looking for.
I'd say your bet best is a chip, freeflow exhaust (not really Powerflow as they are just a franchise and not flow tested) and an induction kit. Most people do not go any further than this as further changes really mean upwards of £1000. Besides, the standard cams in these engines really are quite good. It was after all, the most powerfull 2 litre 8 valve naturally aspirated production engine!
Other than that, the teddy gets it!