It's hard to see in the picture, but there is a V shaped brace in the front bulkhead. You can click on the picture to see a larger version. This version of the model is the working version for my own car. I've been trying to keep accurate models for each of the cars being built, so there are differences. This one, just like my car on the table, is not tidied up yet.
I guess you could make the engine bay diagonals removable, but there seems to be a lot of room. That may change when I actually have to work on the engine though.
Tom has been able to put in and remove his engine with transmission attached a couple of times now and his engine is a couple of inches larger than my Ford Windsor.
I would be a little concerned about making them removable though...
Also to better answer your question, there are significant loads on those diagonals at the front bulkhead. We also try to provide significant stiffness in the dashboard hoop area of the frame by using a vertically oriented 1"x2" in the floor there and providing inner tubes to create an inner hoop on the sides of the cockpit.