This is mostly for those building the big Revell U-boat.
The Engine Room is available, and, it's as nice as the torpedo and control rooms.
You will have to build the engine valve oil feed lines (6) and the pushrods (12 long & 12 Short) but that's a minor annoyance, at worst.
If you plan to do this kit (and still haven't glued the hull halves together), don't plan on gluing them together real soon. Scuttle Butt has it, that, the forward and after batteries are on the way. I guess the after torpedo room will be released after that.
If this means you'll have some free time, on your hands, and you want to do a Guppy, check out Iron Bottom Sound's Guppy Sail kits, for Lionfish. They have a large enough selection that you can probably build any Guppy you want, at, almost, any point in time.
Between Nautilus' WWII conning towers and decks, and Iron Bottom Sound's Guppy sails, you could, probably, build every variation, of a particular boat. From when she was first Commissioned to her final decommissioning.