Lucky is the owner with access to the top of ANY tank, such as you apparently have! Now, on to your solicitation of opinion regarding repair or replace.
Judging from appearance, your tank is aluminum. If so, and it's currently leaking, and 30 years old, I believe the tank has reached end of service life, and should be replaced. There are less-expensive ways to solve your (current) leak, but the "fix" will be far from permanent. You can butch up a repair with all kinds of gak. JB Weld, 5200, duct tape, etc etc. And that's all I'll say about that!
If your (again, presumed) aluminum tank has been routinely filled from city water, most likely that water has been chlorinated. If so, that chlorine has reacted with the aluminum alloy, forming several oxides on the inside. One of them, aluminum oxide, forms hard "crusticles" on the internal walls and floor. Under each crusticle lies a corrosion pit, where the oxide "mines" its contents. Once that pit breaches the wall, the tank leaks.
Welding that hole from the outside may be possible, depending on where the leak site is, how clean the welder can get the weld site, how much "junk" remains inside, etc. It is very unlikely that any welder will attempt a weld repair, even on a water tank, inside a boat hull. Given you can get to the top already, I would pop in an access port immediately (
Seabuilt - Access Plate Systems), inspect the interior, and go from there.
Plastic tanks work fine for water. Finding one that fits is another story, although Ronco (
Ronco Plastics - Marine Water Tanks, RV Water Tank, Auto Detail Tanks, Water Tanks) apparently has several hundred sizes available. Another very good alternative for a custom tank replacement is 316L stainless, from a competent tank manufacturer. If you go the custom tank route, 316L is my personal choice for water tanks, with due diligence regarding design and installation of same.
Regards,
Pete