To be honest, I think that you will find purchasing a new engine will cost you around the same as purchasing a whole new lawnmower. If you are able to find an engine second hand or through a cheap seller, however, I'd say go for it if you really like the old mowers you're using.
Agree with the comment about Honda engines; they are incredibly fuel efficient and don't actually have too many emissions. One of these mowers would definitely be much healthier for you (and the environment!) to be around.