It's very uncommon that kittens as young as Maynard die of FIP. Without a postmortem you can't tell if it was FIP that caused the death of one or both of your cats. The symptoms of FIP resembles many other diseases since the symptoms depends on what tissue the FIP-virus attack. Fever, distended belly, diarrhoea, vomiting, heart failure, bleedings in the eyes, loss of appetite and weight loss are all examples of FIP symptoms and of many other diseases so there aren't any specific FIP symptoms. A postmortem has to be done.

What I can tell you is that FIP isn't contagious. FIP is caused by the FIP-virus (FIPV) and the FIPV is a mutated form of the Feline Enteric Corona Virus (FECoV). FECoV is a very common virus in cats and usually completely harmless. About 80% of all purebred cats have been infected with FECoV but inly about 5% of them will go on and develop FIP. The mutation of FECoV to FIPV inside the cat and the mutation is trigger by stress and in about 50% of the cases due to a genetic predisposition. Once mutated the virus isn't contagious. FECoV is highly contagious (spreads mainly through the litter box) but like I said earlier, it's usually harmless. FIPV isn't contagious but unfortunately fatal.

You could have your current cat tested for the FECoV but that won't help you since a test positive for FECoV doesn't mean that your cat has FIP. As long as he's healthy I wouldn't worry about him, but if you really believe that your other cats died due to FIP you might wanna wait a while before you bing another cat home. Throw away the old litter tray and clean your home thoroughly and wait for at least 6 weeks until you get a new cat. The FIPV is highly sensitive to common desinfectants so there's no advanced cleaning needed and it doesn't survive for long outside its host.