Generally, when a latex item gets damaged it's hard to fix. (I'm convinced that all this latex stuff is just not made to last.) That foam is compressed and once the outside is damaged, that is always going to be a weak spot where it will want to expand and keep breaking. In the past, we've done a combination of gauze type bandages with liquid latex and then paint to match but this can be complicated, time consuming, and you need some skills to make it look good. Just putting some glue or other filler in there can act like a band aid - a temporary fix - but it's probably going to come back apart, especially if you try to force the edges back together.
I'll be looking forward to replies from anyone else who has dealt with this in the past for more suggestions.
Hello Doc Doom. #1 Latex is not made to last. It is an organic compound (tree sap) and it does breakdown over time. Sorry, just the way it is.
#2 This is what I recommend to patch your latex. Being a prop and not a mask makes it a little trickier pretty much working from one side but here we go.
As with any latex product, a straight line will tear again easily. It is best if you can use a hole punch or scissors to make circles at the ends to prevent future tearing.
You will need alcohol (not to drink!, isopropyl), rubber cement, tape, latex, pantyhose or cheesecloth, and something to thicken the latex (toilet paper or cornstarch work)
Clean the area with the alcohol and allow it to dry. Wipe it down with water and allow that to dry.
If possible, try to get a piece of tape inside the prop facing up to help hold the tear together (this is the tricky part with a prop, much easier with a mask).
Try to get the tear as closed as possible.
Apply the rubber cement to the tear and about an inch of the surrounding area.
Place a piece of the pantyhose or cheesecloth over the glued area. Try to keep it smooth if possible (or matching the contours of the prop). Let it dry.
Mix your latex with a thickener. Either tiny pieces of toilet paper or a little cornstarch. You want to make a thick paste-like patching compound.
Spread this compound over the patched area. Use a popsicle stick or other tool to spread it. This will be the final look so get it the way you want it. Blend the edges, add texture, whatever the look you are trying to achieve. If you go thick it takes longer to dry. If you go over it a second time it is best to let the first coat completely dry/cure.
All that is left is to paint the prop and you should be good.
I cannot take full credit for this as I learned from the one and only Allen Hopps. I hope this helps and good luck on the repair.