POR-15 might be your gas tank solution if you are one that likes to do things yourself. I used it for my ‘67 R50.
They have a tank stripper that should remove the lining. Etch for the interior metal and sealant/liner for the final sealant.

Good news and bad news. Good news is I got the inside of the tank clean. Added blue masonry screws, some 3" nails, and a box of bolt nuts to the tank. Then added a small amount of acetone as well as an industrial degreaser. Shook it around a couple times a day and added more liquid when it dried up. Finally got the power washer out yesterday and blew it all out. Bad news is the front brake is locked up solid. Brake handle will not budge, not even a little bit. I disconnected the brake line at the handle and the piston is okay. Can't get any fluid out of either side of the caliper bleeders. Not sure how to proceed now.

Could be a clogged line, disconnect it at both ends and blow air through it to see if its clean.

I would remove all the fluid from the MC flush some new fluid through it and the clear line.

You can also remove the caliper drain it move the pistons in and out manually or with some gentle air, if you put too much air it can pop them out.

Once the line is clear and the pistons move, fill the MC and bleed the system.

If the pistons are stuck you may have to take the caliper apart and order a rebuild kit.