playing around with the simulator, there are a lot of different sources of errors, but the first one to try and address is the distance between the motors. An error of 2mm here can result in almost a 5mm error in the final cuts.
We measure this by using the chain, which is FAR more accurate than trying to use a tape measure.
However, this means that any flex in the frame that changes this distance i very significant. The stock frame depends on the plywood not warping, the arms not flexing, the screws holding the arms in place having no play, the screws holding the motor mount plywood into place having no play, the screws holding the motors in place having no play, and none of the wood pieces changing dimensions significantly due to humidity.
That seems to me to be far too many variables, with far too many different angles for something that’s so sensitive.
So I am thinking that the first thing to do is to eliminate most of those variables by using a solid beam across the top to separate the motors.
Starting with a 10’ long 2x10 beam (ideally LVL, but a good, straight board of other wood is probably ok, and still far better than the stock design), if you mount the motors directly to this beam, you have eliminated as many of the variables as you possibly can (it’s also worth considering using a hunk of metal for this, although I’d want to look at the coefficient of expansion for the metal in questions)
From this starting point, the rest of the frame dimensions are not critical. You need to support the beam in place at an angle to match your workpiece, support the workpiece, and keep the chains from tangling
So I am thinking that the frame would have a 8-10 foot lower rail (2x2 or 2x4), to support the workpiece and chain ends mounted to a couple of legs. The legs would be a bit taller than the stock ones (6.5 ft or so) so that they extend above the workpiece to support the upper beam.
I would put a 2x4 between the legs and bolt the beam to this 2x4 so that the beam could be moved in and out as needed to keep the chains parallel to the workpiece if you are dealing with especially thick or thin material
The legs can either be a-frames (like the stock ones, just a bit taller), or you can hinge the top of the frame to the wall/rafters
The stiffness of the main part of the frame should not be critical, so a simple 3/4 MDF/plywood board against a pair of legs is probably good enough, but adding some lightweight framing in a grid (say 1/2" thick) and attaching a 1/4" piece of plywood to the back would make a very stiff torsion box.
 note that a 2x lower rail will work great if you are cutting 3/4 material, but if you are going to cut 1/2" or 1/4" material, the sled will catch on the lower rail, so you may want to make it thinner or have some provision to easily pull it off if you need to work with thin material)