Er, I don't think the later "fat" radiator grille will fit an earlier tractor straight off. I presume you mean the grille for the HST version of the B7100? (the geared version has a different grille)
The earlier "corrugated" grille for the HST fitted directly to some lugs which were on the sides of the radiator but the later "fat" grille has to fit to some additional hardware. I know this because I bought a second-hand "fat" grille but found the hardware to make it fit was so expensive the grille is now hanging on my wall a few feet from where I am typing this.
I have found a couple of new old stock earlier grilles on the 'bay... You could try a salvage yard but the thin lugs on the grilles are very prone to rust.
Good luck!
Curly