Stated above is the practical solution.
As an example: The damaged tunnel area is 7 inches long, and extends laterally into the flat portion of the floor for about 2 inches on each side of the tunnel.
From a car in a wrecking yard a matching piece would be cut that is about 9 inches long, and 3-4 inches out on each side.
After trimming to give an overlap of about 1 inch all around, the piece can be attached on the inside of the tunnel/floor via riveting and bonding with structural adhesive.

That is a repair that a lot of fairly common people can do with the car on jack stands.

Update:

The problem is this transmission tunnel removal requires a lot of parts to be set aside to gain access to the transmission tunnel. In fact, the mbz parts website gave me some excellent pointers. Here is her response:

I would try searching Car-Part.com and ask if anyone has a W124 shell. That's the key. We have two W124 cars but they have all their interior
parts still in them, so you want to find someone who has already stripped the interiors out and can inspect the transmission tunnel without having to pull seats and carpeting and the console.

This is where I am right now. So far, I tried this car-parts.com and I am still not sure what category to use to find this transmission tunnel part. So I used transmission category and contact through live chat and sending emails. Some replied and others did not. All said the same thing , meaning they don't have a w124 shell by itself and/or not willing to remove parts to gain access to the transmission tunnel. In addition, possible sources of locating this part is in salvage yards, even though most salvage yards refuse to separate the transmission tunnel from the chassis and basically scrapped it.

I think the bottom line is this: Unless I get a w124 shell access alone by itself/transmission tunnel, I would probably have to go to pull a part junk yard. That means having pull seats, carpeting and console to gain access to the transmission tunnel. After that, I probably have to use a lithium battery powered 5" angle grinder with a couple cutoff disks.

I am trying to avoid going to a junk yard that is 60 miles away from home.

Any comments, feedback/advice is greatly appreciated.

Go to a chassis builder or fabricator/welder, they will make and install what is needed. You could do much of the prep work by removing seats, console and carpet.

Unless this is a show car, searching out an OE replacement part is respectfully a little "over-the-top" and could make the difference of whether or not the car gets back on the road.

Good luck!!!

Do you have any pictures of the tunnel damage you can share?

Hirnbeiss, . I do not have pictures of the damaged areas.

Sugar Bear, your suggestions is greatly appreciated. Stay tune.

cutting out and welding in a replacement transmission tunnel is a monumental task as its a stressed member of the body. A full replacement would get very expensive rather quickly.

How bad is yours damaged? - have you visited any other body repair shops? Repair of the current one in place is quite easily possible unless yours is buckled to the point of the car being scrapped.

Zulfiqar, thank you for your post. I don't know or can't tell how badly damaged in regards to the center floor pan/transmission tunnel. All I know is that I need a complete replacement according to the shop foreman and I agreed with his assessment. At this time, I am still researching this replacement transmission tunnel. Stay tune.

This is turning out to be more complicated and drama filled than it needs to be.

Post pictures of the damage. To get this fixed, pull all seats / rug , reinstall driver seat, take to body shop for an estimate.

They will either tell you that they can weld patches over the torn area or that you need to get a piece cut out of another car.

Patching from the interior side then sealing both sides with body seam sealer is the most direct route to repairs. This might take 3 hr including some sort of paint on both sides if they start with a bare interior.

Also, the salvage yard search is car-part.com ( not partS ) others are not the same.

In your place, I'd remove the center console and carpet and take a good look. I'll bet the damaged parts can be beaten down back into position, and then the cuts welded closed. With the interior parts out of the way, it should be obvious if this will really require new metal to replace old, or a more basic repair.

Check your local used car listings to see if anyone is selling a 124 for parts, maybe leave your contact info in case it is purchased locally and then you can make a deal with the new owner to cut out the tranny tunnel after they remove the interior or whatever. Have a stripping party, invite your friends and help the new owner take the car apart down to the shell, there are many ways to skin this cat they just take some time.

The post is useless without pics.