I'm so sorry for your loss, it's no doubt one of the hardest things you will ever do, but you did it out of love. Joey does not suffer now.
IF you decided to get a new cat/kitten through the SPCA or some other adoption place (or a home) maybe find out if you can bring your pup to meet the cat first, to see if they react well to each other. When I adopted Halo, our older dog Shadow was a factor in the decision.
Like Longblades said though, I wouldn't rush it.