I saw in a YouTube video recently (from King of Random, he was making a mold of a roast chicken) that casting silicone doesn't stick to anything except silicone rubber (he discovered the exception at the loss of a silicone spatula). That being the case, you may be able to get some silicone intended for making molds (for casting resin or low-melting metals) and use that to stick your torn silicone part together.
If the tear in isn't in an area that has to stretch in use, I'd suggest also adhering on a layer of nylon or fiberglass mesh as reinforcement, since the repair is likely to be weaker than the original part, which has already failed.