Mustang suppliers (MustangsUnlimited, NPD, etc.) sell a replacement temp gauge but as Veronica mentioned you'd want to see if it's the sending unit in the intake manifold that's bad first (check cheapest fix first).
I have not seen the full size tachs ~5" that replace the gas gauge pod for sale, I think someone was making one but it was $$$ like >$200? These came on Shelby's and special cars, usally big blocks with drag packs, etc. so they're very hard to find original and way more than the repops.
The small retrogauge tach fits in the center clock/blank hole, I have one for my automatic car and it works great/looks stock as it has the same fonts as the other gauges.
'67 Coupe 289 stroked to 333, AOD, 9" w/3.50:1, PDB, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon