You're looking at 40 years of degradation to the - every (rubber, for the time) seal needing to be replaced, in every fitting and valve. - the lower unit seem

You're looking at 40 years of degradation to the - every (rubber, for the time) seal needing to be replaced, in every fitting and valve. - the lower unit seems to have fractured under its own weight (this is likely the ballast tanks), this would need to be remade. - any pressure areas need to be retested (including the crew area) and replaced (all compressed air storage). Now, for the easy parts - the batteries will have to be replaced, but lithium ion is going to give you better performance at smaller size, with less degradation. (You'll have to compensate the weight though) - the motors will have to be replaced, but can use much more efficient, likely better sealed, better cooled, smaller, safer motors with pwm electric speed controllers. - modern glass and fastening techniques could replace the viewports, possibly add more as you rebuild the negative pressure vessel. - air management is easier to instrument. - you can add a bunch more cameras and sensors on the cheap that are significantly better than the let's