I cleaned and rebuilt one of these as a feasibility test - not worth the effort due to the hours of labour needed.
The only place I found the gasket advertised was Brazil, it’s NBR coated stainless steel.
I didn’t try and purchase as I had no plans to use it anyway.
I rebuilt the unit using the old gasket with a thin coating of Rocol Steamseal paste, good for a max’ of 600 deg C.
I pressure tested the gas and water side and no leaks.
Sitting in my workshop as a ‘paper weight’.
If the tubes are full of carbon you can’t get the chemical through, I needed to drill out each one with a long series drill bit, PITA. The header is also going to be solid with hard carbon.