Glad to see progress. looking good Casey
Elevation: I would do what your thinking, but consider spacers on the end holes inside the tubes
Air filter: I'd leave it unless it will rattle.
Choke: should be just a bracket to hold the cable?
Was already thinking I would need to cut spacers, thinking about 6 of them out of 3/8" pipe, unless I'm dealing with 7/16 bolts and didn't notice lol. I am considering the possibility of running a section of box under the engine plate to help stabilize, but nothing certain, and probably wont unless I see an issue. Only concern is my engine is all to one side, as opposed to even distribution across the plate (Briggs opposed had bigger footprint and was centered).
Air filter is in my mind as I want to leave but my hood hinges are the issue, Mabey.
Exhaust, I have an idea right now to take the old muffler off the top, make an adaper to stick one of those Gravely/Kohler mufflers on it to aim it out the headlight hole (diagonally). Shouldn't add enough weight to require additional bracing, which is good, if it works out, and hope to get it quieter that way.
I have yet to look at the connections to the engine, as it is secondary to the install.