You still seem to be missing a big point of the whole project.
You shouldn't need to go out and find the design somewhere that someone else did and then just copy it. That, indeed, will cheat you out of the educational opportunity.
But NOT using the chips that YOU have designed also cheats you out of an educational opportunity.
You mention that you learned a lot by creating the 16-bit adder chip. That chip, which you understand how it works inside, should now become a building block at that level as you make more complex designs. If you need to add two 16-bit values, use your 16-bit adder chip. Don't fall back and wire up 32 more primitive chips. If you are going to do that, why use And and Xor gates? Why let yourself use Mux gates? Why not go all the way down to the Nand and DFF gates that everything reduces to in the end? What's the point? It adds nothing to your further understanding -- in fact it likely makes it harder to learn the bigger-picture understanding of how the more complex circuit works at a functional level.
If you feel the need (or desire) to use Logisim for your simulations instead of the provided hardware simulator, then learn how to create subcircuits in Logisim so that you can create comparable parts, such as a 16-bit adder, and use those as your building blocks.
But, leaving all of that aside, you still have the issue I point out with your RESET logic (and which is also present in other places in your design).