Mosrt of these parts are available, though they won't be a perfect match...in terms of collectors.
You can order Sunn Tolex from Fender! The grille cloth you might be able to find. It's similar to Fender's "blackface" cloth, but it IS differant. Acoustone uesd to handle it.
You are taking on quite a labor-intensive project. I'verestored a dozen Sunn cabinets, and even built a few from scratch, and it's quite a job!
Where to begin...?
You can fill the cut-out area on the back, as stock Sunn 2-15 cabinets had 2 recessed cups for 1/4" female jacks. It had either one handle centered on one side, or 2 handles at the top and bottom of the same side...depending on the year of manufacture. If you look inside you should see the holes. When I re-habed my 2-15s, I put one handle in the center and one handle at the top of the same side, and used T nuts. I did these this way because 1) I had covers that had cut-outs for the center handle and 2) if 2 people are available to carry these, then one can grab the top/side handle and the other can grab the wheels. I have some parts and know of sourses for almost all of the hardware needed to restore these.
Anyway, as you can see, this can get very involved.
e-mail me, or post here, if you want other help or advice.
Welcome to the "Save the Sunn" group!
Tom