Roots,
I think .010" would be a pretty safe number for a Fox Stunt. That leaves room for an oil film. This would be when the crank is pushed all the way back and a prop installed.* You probably only need 2-3 thousandths--but since it is a Fox Stunt, it might be hard to ensure that happens. Note that you'll have to account for the backplate gasket. It looked from the pictures that you are using the Fox cork part? This will compress over time and reduce your clearance. Paper gaskets are more predictable, but generally thinner, so again you would have to pick one and should stick with it. If you are familiar with OS engines, they use what is often called "vegetable paper." Just a natural fiber paper that works well for gaskets. I've always assumed that the reason Fox used cork was that because it can compress much farther, it could take up larger non-uniformities in either of the surfaces of the parts. I've seen cast backplates with pretty large cold shuts which otherwise would have leaked. I've seen the same thing on some McCoy Redheads of the same era.
If you do find a so-called "stuffer" backplate**, be aware that these were made with more precision (for the anniversary cases) and much tighter fits than the old cast parts, which had enormous diametral clearance. I just resuscitated a number of old Fox Stunts that a club member had collected and run hard. In most cases, really hard. He came to me because he could not get the stuffer backplate out of the older Fox cases. The various different engine reworkers had tried to remove the internal mold flash and grind the ID a bit rounder, but in three engines, the backplates were really seized internally. They must have pressed the backplate in, heat shrunk it in, or maybe just forced it on with the screws. The metal had galled and...well, you get the idea. As you have an earlier case, be prepared to do some work if needed to get the barstock part in.
Another way to confirm clearance is to put down multiple layers of tape on the face of the backplate. Reassemble the engine and slowly seat the screws while dithering the crank back and forth (rotation). When you feel added drag, you're touching. Take some tape out and keep trying. Then measure the tape. This kind of in-situ measurement can either confirm parts measurements, or account for unknown gasket compression. You also don't need five hands to hold the case, push the crank all the way in, and hold a depth mike or calipers without fumbling the whole deal onto the concrete floor....
One other thought since you are into tweaking this engine: if you can't find a barstock backplate, and if you have a ton of end clearance, consider making an aluminum plate and bonding it onto your current backplate. If you got the parts thoroughly clean and used JB Weld it should work fine. If you make the plate diameter oversize, you can dress it after the bond cures. Something like 6061-T6 would work nicely.
The Divot
*--If you have a ton of shaft endplay with the prop on and the backplate off, you might want to put a precision shaft spacer between the drive hub and the case. Or a washer, if you have one thin enough.
**--The real value of the barstock backplate is in the much reduced end clearance, saving your rod and crank, and not in attempting to fill the huge internal volume of the Fox and improve base compression.