According to Jamil in brewing classic styles simcoe is in the group with [BC-Goldings, glacier, Chinook,cluster ] so according to that, chinook would be a good substitute, but I'd say it's a little too distinctive of a hop to use as a substitute, even if used just for bittering I find Chinook has a way of still coming through with its flavor and aroma. you might be better off using cluster for bittering, then a combination of late addition hops to try and get that simcoe character. Amarillo for the citrus side, glacier for the "dank" and some Norther brewer for spice.
Simcoe is vary unique, I like simcoe a lot and would try hard to find some before subbing it.