From looking at the datasheets, it appears that it would be acceptable (same pinout, slightly higher specs at 650V vs. 600V, same current). The picture on Mouser's site shows a 3 lead TO-220 package for the IDP part, but the datasheet shows a 2 lead TO220, which is what the ISL part is. I'm no expert on SMPS however, so take that as it is.