Not terrifying at all.. for the most part Fortran-used-like-C looks pretty much like... C. Declare a header signature the right way, call any Fortran function from C, declare any C function with an underscore at the end and constrained to some rules and call it from Fortran. Like other comments point out, there’s nothing “ancient” about the language. You don’t need to hunt for dusty floppies and emulators to boot old OS images from tapes. GCC 9.2 compiles it all.