My brothers was bolted back on when he replaced it last. it held up as good as the rivets. a little more time consuming to put it back together, but if you think you will have to replace again, might not be a bad idea. personally i dont think drilling the rivets is too bad (once they aren't the stock ones)
can't remember for sure, but dont think you have to remove engine.
to remove the steel rivets go buy a GOOD drill bit or two and go to town. they are a pain, but no other way to do it really. air chisel maybe would work. center punch the round headed ones helps too.
i think Skidoo has a bulletin on replacing the nun and what bolts and everything to use. I am at work right now, but will see if I have it on my comp at home when I get a chance.