Metal steering lines leaking

Plastic or Pex is a No No.
There is a synthetic tube that is available but not recommended by me.
Copper or hydrolic hose is where you want to go.
Light copper tubing will work but when I have to use Copper in a line like this, I purchase the heavier A/C Copper tubing. Thicker wall, takes more pressure, takes more abuse (what killed your tubing) and better seal at the flair. Just over twice the price but still cheap.

Now, for the fix;
The copper lines comes from the helm(s) to the stern. They would terminate to a alloy block, then hoses to the steering ram.
How far from the block is your tubing damage?

Can you offer diagrams, measurements between connects and pictures of the area the tube is leaking, the alloy block and steering ram hoses.

Please confirm it's a Hynautic steering system. It has a bottle somewhere with w air gauge on top.
Or that other ole brand that you fill at the top helm head and ad no pressure.

Do I sense a reluctance in you flaring copper tubing?

I inspected the copper tubing when I repowered and rebuild the steering and it was still in excellent condition. Original since 1970 but I think Hatteras used thick wall tubes.

Braided hose can be used but costly if you have long run. Or maybe plastic tubing like hynautic uses in outboard systems. Check their site for recommendation.

If you only have one leak and rest looks fine, just repair it with a new section

I get things like this from Untied Refrigeration Supply or Grainger....etc...the tubing comes in a roll. They have the flair tool and tubing cutter. You can do the work it's not to hard. trace the entire line , the best you can first and look for more leaks just in case. It's not to hard just to run one new complete section in from point A to B. That would be the right way.

To much hidden copper that is probably still in great shape.
I have re-plumbed boats before (including mine) in the past for the steering system. More cost in materials but cheaper in labor; run hose.
Lets get more data from Boomer before we start throwing any materials or money at his issue.

True, it is a long, tricky run from the bridge on down...well my 1981 system is still holding pressure and no leaks....yet...tick,tick,tick tock, pop! My time will come....