I tried some of that commercially available rust converter on a firebox, and wouldn't do it again.
If I found a gem like that, I would invest in a thorough sand/media blasting and give it a good paint job outside and re-season the inside. Have some of the "too far gone" pieces fabricated new.
Then stay on top of keeping it painted, especially the firebox. I think some Lang owners hit the exterior of the firebox with some kind of fat after each cook, but I may be wrong.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 11 WSMs (B, C, E, DH, DA, EZ, DZ, EO, DU, DR, EE/DU); 5 - 22.5" Bar-B-Q-Kettles; 2 - 18.5" Bar-B-Q-Kettle; 3 WGAs (EZ, DE, N); 2 SJPs ( DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU); Brinkman Cimmaron; Chuckwagon Cooker; .
"For when democracy becomes tyranny, those of us with rifles still get to vote"