You (meaning the average "dauber" welder) DO NOT EVEN THINK ABOUT welding on a pressure vessel. Anyone who does it for you (especially for cash money) has his ass hanging out a mile and a half for liability - this is a specialized field with lots of pitfalls.
If you really want it fixed, you need a certified boiler and pressure vessel welder to do the work. They do exist, these are the same people who weld in new crown sheets and repair patches on old steam engine boilers and large stationary boilers.
They have to match the type and gauge steel used in the tank, roll the patch to the proper curves, use the proper welding wire or rod, pre and post heat treat as needed, repair any staybolts or attachments as needed, X-ray or otherwise check their work (and for any other latent problems they might have missed) and then hydrotest.
But a brand new factory built tank is probably cheaper than paying someone to repair and recertify the one you have. And this time, mount the compressor remotely.
The reason they spend a lot of money to repair those historic boilers is repair is still cheaper than building a new boiler to the exact old dimensions from scratch - and then rebuilding the entire vehicle around it. Or in the case of a huge power plant, wrecking the old and erecting an all-new boiler in the same space - they don't ship well when you get up in the million pound range...
Or they don't want to destroy any 'historic value' of a steam engine with an all new duplicate boiler, which is silly if they duplicate it faithfully (well, except for the lap-seams and rivets...) and they reuse the chassis and all the old running gear.
There are times when safety trumps perfect historical accuracy, and this is one of them - the aftermath of tanks and boilers going BOOM!! is not pretty.
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