If you break one of your idler pulleys in a Subaru will this wreck your engine?

There's two different engines available and both are "interference" engines so if the timing belt jumped a few teeth the valves will be damaged. Most people don't replace the entire engine, they have a valve job done along with the new timing belt and head gasket. If you put in a used engine from the salvage yard, you're just getting another one that might need a new belt soon too.