Sensor Jammer changes the opponent's HIT result to an EYE. It says you cant reroll to change the result of that die, but can they use calculation to change it to a CRIT?

Yes - or use a focus token to change it to a hit

the first part of the attack sequence goes:

attacker rolls dice

Defender modifes attack dice - this is where sensor jammer happens

attacker modifies attack dice - this is where TLs, focus etc are spent

As noted Calculation is not a "re-roll" but simply a modification that happens after the defender makes his modification.