The biggest probblem with the Stuart is its size. It is incredibly small inside. Normal 21st century people simply do not fit inside. Peoiple that were 5'10" and 150 lb found it small. Strapping modern troopers pushing 6'2" and 220 of muscle simply will not fit inside unless you make significant structural changes.
If you want to "modernize" it, about the best you can do is what the Brazilians did in the early 70s:
If you are willing to virtually make a complete new vehicle you can get as far as this:
Either of those vehicles _could_ have image intensification systems and maybe simplistic laser rangefinders and then you could add ERA or ceramic slab armour, but the suspensions would be brutally overloaded.
If you go with a new turret with larger turret ring, then the gun can be changed to somehithg more suitable to the mission and for anti smuggling patrols the 30mm Rarden from the CVRT series of tanks is a good choice. Lots of ammo choices and a feed system that can swap between two types simultaneously loaded in the system. Also optimised for AFV use so it's not chewing through ammo like an aircraft autocannon (
).
Change the engine out for a modern diesel and add an IR suppression exhaust system and a turret bustle for radios & electronics.
Just a few thoughts,
Paul