Yes, this is a tough one. I'm pretty sure I remember that GE had that model made for them by White Westinghouse. The company was sold a couple of times and ended up be owned by Frigidaire (now Electrolux).
So, I went there first hoping that they still have that size. Although I could not find an exact fit, I did find I believe the closes fit.
You would need to cut 2 more inches on the sides (an inch on each side maybe) for the width. You would also add a piece to cut the front to back dimension down. I have seen a number of cabinet makers do this sort of adjustment without too much difficulty.
If you go to this link below, you will see the model that I think is your best fit.
If you already have considered this model, let me know what other manufacturers and/or models that you have already checked.
We really wish to avoid cutting into tile counter top which will also adversely affect the drawers and cabinets located under the cooktop. Are you aware of any vendors that may have these discontinued G.E. cooktops in their stock?. Are the burners and electric starters for these models available from any source? XXXXX XXXXX
You would have to try "used" appliance dealers to see if you can find that model with one of them. No one would have a model that old (unless it was some very unusual fluke). Appliances dealers must move old inventory or go out of business. Having a model more than 3 years old is a real problem for an appliance dealer.
Try the link below for parts. If they are not able to supply the parts, they may be able to tell you where you can get the parts or how you can substitute other parts for those. I have to believe that there would be substitutes for them.
I looked for a 30" version of the above cooktop from Frigidaire and I could not find one.
That's the most shallow I could find.