There are a lot of "how to make a sewing machine cake" articles out there, but I couldn't find one that showed the internal structure and wasn't made of rice krispies. I made a sewing machine cake recently and took photos beforehand of the structural parts of it.
The electronic machines today are different from older models, in that the better machines are pretty enclosed. The tops are smooth and the thread is all on the inside. The last two cakes that I've made were specific brands, so they don't have the thread spool on the top of the cake. If you wanted to change to one of the older models you could just add the spool on top, the basic internal supports would stay the same.
The basic structure is cake on the base, cake on the side, cake on the top, and styrofoam or something else for the needle housing part. Since that's the part that hangs down it would be structurally unstable to have that made from cake too. You can attach the non-cake part either by hot gluing it to the bottom of the board, or by wrapping it with tape, which is what I did.
So I'm going to give you the internal structure, and you can take it from there.
Start with two greaseproof support boards, both 13" long. One should be 6" wide and the other about 4 1/2" wide.
Get the layers stacked, then cut them off level with the plastic dowels. The top support will sit right on top of that and the front leg should support the board on the left and make a flat surface for the top cake. Put a couple of wooden dowels into the cake to provide support between the top board and the 5x6" board that supports the cake on the right side.
So that's the internal structure, and you can take it from there, If you wanted to cover the styrofoam section with fondant or modeling chocolate BEFORE putting the upper cake on it it might be easier to handle in the long run, but you can do it either way.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA