I've thought about an updated post WWII Comet a few times. My take:
1. Lose the bow MG and driver's direct vision hatch. Fit a slab of sloping armour in their place, roughly copying the external lines of the Centurion. Give the driver modern periscopes to compensate and use the bow MG gunner's space for extra storage.
2. Fit WWII German-style spaced armour outboard of the tracks and around the turret, in a scheme generally remeniscent of a late Pzkpw IV. This is mainly an anti-RPG screen, since shaped charges were the latest new threat to emerge in the timeframe.
3. Fit a modern commander's cupola with a remote-controlled MG.
4. Replace the 7.92mm BESA MG with a 7.62mm equivent IF that produces a logistics advantage.
5. I'm torn about replacing the engine. On the one hand, it's petrol, which is a generally bad idea, but on the other it was powerful and reliable, so maybe better the devil you know etc...? There's also the issue of total cost: tank rebuilds can end up like Trigger's broom* if you're not careful.
6. One big and not easily fixed (in the 1950s) problem with the Comet was weak belly armour, making it particularly vulnerable to mines. This would be particularly significant in a Vietnam-like COIN scenario. All I can see to do with that one is add some applique to the underside, but you're have to be careful about ground clearance. Maybe the suspension could be modified to compensate but there again, Trigger's Broom....
Incidentally Finland had Comets in war-reserve stocks until 2007!
*For the uninitiated, Trigger's Broom was from an episode of the UK sitcom Only Fools and Horses. Trigger (whose nickname is ironic: he's actually not very bright) remarks that he's had the same broom for 20 years, and it's been perfectly reliable, only needing 17 new heads and 14 new handles in all that time. It's a modern version of the Ship Of Theseus paradox, i.e. if an object has gradually had every component replaced by new ones, is it still the same old object or a new one?