The closest thing you will find is a Dynacorn body...but they are $15000+ for the shell with no interior, suspension, bolt on sheet metal, or any of the many other things that make up a car.
If you want a replica your best best is to buy a fiberglass body, I don't know if anyone makes the '69 body style, but you can get the 67-68:
Get a used 69FB mustang and put the scoop and stickers on it with the long center cap rims and your there.
For an engine Jon Kaass sells Ford Hemi Heads for a 460 block. You can make your own 'Boss 9'.
There are after market perimeter frames to go under mustangs and can hold lots of power, big tires, and ect.
Sheet metal is available to make a coupe a fast bk.
This all very doable and has been done before but is not a 'kit'
People have even made a "One of None" factory 'what if' convertible.