Hi Buddy,
How wide is the narrowest corner for the top run on a 45° from the corner?
What's the height difference between levels? (especially the top and the middle)
What's the clearance from the wall on the long straights on the top level?
What track and switches are you using?
I fit an Atlas O99 around the top with 3'-3 1/2" from the wall all the way around in a 12' (144") x 22' (264") area. it needs a corner of 31" on a 45° from the corner for the top.
It looks like your drop is around 10" to the next level, so I split the difference going down 5" for a second O90 loop and then down another 5" to an O81 loop. Essentially, add another loop to cut the grades.
This may let you keep the layout structure and provide a level and up and down loop to run the big engines.
I am working on concept to modify the existing structure and keep the grades at 2.5% to travel between levels. Perhaps leave the base level as it is and modify only the middle and top levels to connect and handle the large steamers. The big steamers won't fit on the TT.
I am still working on the grades and such, but here's some 3D of where it is now. I need to work something like a passing siding to keep the train on the center.