Fronts will bolt in. Rears will require you to either swap your Imlreza struts tops over to the Legacy struts or drill a hole so that all 3 studs from the Legacy struts will line up.
Theyll bolt on, but outback springs are softer than Forester springs. It also would not add any lift. Might even lose a slight amount of height using outback suspension on a forester.
Unfortunately a cat is required in Maine. You can either try to find a box road shop that doesn't care, or get a carted downpipe and a retune, and get a sticker. Yoir other option is to just swap a stock downpipe on to get a sticker, and then swap your other one on afterwards.
The one you posted a picture of is an 85-89 style. They made the same body up to 94. I have a couple of them but I don't think I could ever sell. But let me know if you find a decent one. Sadly, most of them are rusted away around here.