Thanks, Chris--
Perhaps I should be more clear: I'm repackaging here, not creating a new setup. And since I'm creating a transform, I won't be running it with the SETUP.EXE, which includes the required engine embedded.
However, I can see from the properties that the MSI in question does in fact use ISScripts and requires version 10.1.0.128.
Standard procedure for installing this (with the custom transform) would be to silently install the ISScript10.msi redistributable first, but I can't locate a copy of the version I need anywhere on the InstallShield site. Another possible option is to include the appropriate merge module and tweak the properties to detect it correctly, but that's a little messier, and AFAIK InstallShield has only given out MSMs thru InstallShield 9sp1.
I was hoping someone on the forums here might have an idea where I could locate the latest versions of these, as InstallShield's official download pages and knowledge base lead you in many directions to many different versions of the redistributables. Ugh...