Won’t quite do the same since it won’t actually accept a source file to check sha1 sums against, but if you just need to generate compatible output, you could try defining the following function in your .bashrc:

sha1sum ()
{
openssl sha1 $1 | sed ‘s/.*((.*))= (.*)$/2 1/’
}

This will, on a “sha1sum foo.txt” generate the expected output…