I get 4.5" from the center rail to the top wall and 3 3/8" from the center rail to the right wall. I would say it's plenty of space for the overhang of any non-scale engines.
I agree with Carl, I believe you have adequate clearance if the measurements in SCARM are accurate enough.
As drawn, the bench work in the upper left begins near grid mark 64.25" on the "X" axis and goes to 231" making the top length 166.75", slightly less than 14'. My guess is it's supposed to be 14'.
The lower side of the entry starts at about 65.75", a 1.5" difference. FYI
In the lower right, it ends at 135.25" on the "Y" axis making the right side length 11' 4". If it's really supposed to be 11', that's a 4" difference and could affect the clearance of that lower curve.
There are 4 pieces of GarGraves "S" gauge flex track in the design. FYI