I highly recommend the Damon Redkote sealer.
It is made for tanks like that. Solder the pinholes and follow the instuctions. Use two coats.
It stays flexible. It’s all I use.
I had a similar situation on my OA55 tank. I soldered the pin holes and lined it. That was 11 years ago and gallons and gallons of gas have been through the tank with no issues.
If used per directions it’s extremely durable.
A quart of it goes a long way.