If I had to do it, I'd do a find/change in a word processor, changing, for example, "End Do" to "Continue" or whatever. It will be tedious, but would work. If you have a 100,000 line program, I feel sorry for you. There's a FORTRAN library at
Not all F90 code can be directly converted to F77. For instance, F90 modules have no counterpart in F77. You'll have to convert those by hand, like it or not. I haven't heard of anything that does what you need ( as it is a step backwards ).
I assume you want this because you have an F77 compiler and not a F90 compiler. You might want to consider using F from Imagine. I believe it is free. Also, ELF90 from Lahey is pretty inexpensive. Also... LF95 standard edition from Lahey is not too expensive. Both F and ELF90 are subsets of F90... so some conversion would be likely , but... not nearly as much as converting all the way back to the F77 standard.
