Carastan is a light English crystal malt that has a toastier, more pronounced toffee flavor than American equivalents like Crystal 20L, 40L, etc. Your addition of Crystal 20L (and maybe the Munich to a degree) pretty much meets the requirements of the original recipe. There's some good info about CaraStan here
and this one here
includes making a mix of American Crystal malts as a substitute. Also, this chart
includes substitutions based on manufacturers.
Here is a description of CaraStan from Bairds:
Bairds Carastan (30-40°L)
Bairds Carastan malt is made through a process of stewing germinated malt followed by an extensive period of high-degree roasting for varying periods of time. The result is a pale, brown-colored malt with a caramel/toffee-like flavor.