Maddii...Welcome to the Daily Puppy. Cavaliers are wonderful dogs and very loving. I must be very frank with you and express that if you are unaware of the ramifications, or lack thereof, of a wee umbilical hernia, you ought not to be considering breeding at this time. I would think that if you are considering breeding, than you must also be considering the show ring.
Most well bred Cavaliers are sold with or without a breeding contract, and if they aren't being sold that way, they are born under suspicious circumstances. Both Oliver and Henry were born with those and they are both fine. They also came from reputable breeders. Depending on the size of the hernia, and whether or not it is a health concern to the pup, will depend on whether or not to repair it with surgery. I had Oliver's repaired when he was neutered, but left Henry's alone when he was also neutered.
Lastly, I do not believe that these Hernia's are hereditary. I believe it has everything to do with what occurs in the womb and being born. However, I do not know that for a fact and I may well be wrong. We have other members on here that know more than I do when it comes to such things, so hopefully one of them will let us know.
While your Cavalier may enjoy a new playmate, please consider the source of any new pup and the reason for acquiring one. Oliver was 5 last summer when we brought Henry home. They are now best friends, but I don't think Oliver would have minded being an "only dog". Good luck to you, and I'm very sorry for the loss of your cat.