Allegedly, the problem with the upper playfield is that it was tooled as a one-piece injection mold. This much IS true: if you examine the part, you can see that it couldn't be simply vacu-formed like a traditional ramp. I believe the vented lamp domes for the lanes are where most of this complexity comes from.
Personally I have to think if the original mold existed somewhere, Planetary would have remade these by now. So let's presume it doesn't. New injection molds are boku bucks.... notgonnahappen.
But with that said, I personally wouldn't mind a vacu-formed replacement that is close enough to be serviceable. I mean if you take the lane domes out, what you're left with is really no more complex than a traditional ramp. Possibly even less so, since it's smaller and flatter. You might have some differences in the joint of the back wall to the floor (rounded instead of square corner) potentially, but that wouldn't affect functionality. Vacu-form the flat platform, with new holes to add traditional lane guides (or maybe Gottlieb's pyramid style so that you can keep the solid ends... or perhaps newly vacu-formed individual lane guides like the original Station ones)... just screw them in. Boom. Done.
Basically if a one-piece is cost-prohibitive, fine. Make it multi-piece. Beggars can't be choosers for original look, but if you want something that works, I think this could, um, work.