Your remaining hen may get lonely on her own, Tuscan, they're flocking birds and need company. The problem with introducing a single hen is usually that the older girl(s) are far from welcoming and there will be a time when there's a lot of unrest and bullying of the newbie. It's better if you can introduce two, and keep them separate for two weeks, partly as quarantine, and also so they can see the other bird(s) through wire mesh and settle down a bit before going in to the main run. If you just put them in together straight away, all hell may break loose. However, we're now coming up to the time of year when this process gets a bit easier, because older hens will be approaching the moult at the end of their breeding season and won't be so aggressive or bothered about newbies. So, if you have room for three in your coop, I would advise getting a pair of replacements rather than a singleton, especially as your remaining girl hasn't been in the best of health and isn't a youngster. With her shorter life expectancy, if you got a couple you could perhaps avoid the problem some time next year of being left again with a single hen.
You mention rehoming an unwanted hen of similar age or an exbatt. I think you'd be much better off, if space permits, getting a couple of young point-of-lay hybrid pullets, who could be expected to go on together for a longer time, and would come into lay before the end of September, providing a steady stream of eggs all through their first winter, when the older hen will be resting. Rehoming older hens, or exbatts, will most probably bring you all sorts of unseen health problems, from imported parasites or bacterial infections on an older hen from a different flock, to egg laying, prolapse, tumours and peritonitis which are unfortunately very common in exbattery hens. I can understand the feel-good factor of rehoming, but if you want a better chance of perhaps 3-5 relatively trouble-free years with younger birds, they will respond to your excellent care and will probably be less stressful for you in the long run.
You mention rehoming an unwanted hen of similar age or an exbatt. I think you'd be much better off, if space permits, getting a couple of young point-of-lay hybrid pullets, who could be expected to go on together for a longer time, and would come into lay before the end of September, providing a steady stream of eggs all through their first winter, when the older hen will be resting. Rehoming older hens, or exbatts, will most probably bring you all sorts of unseen health problems, from imported parasites or bacterial infections on an older hen from a different flock, to egg laying, prolapse, tumours and peritonitis which are unfortunately very common in exbattery hens. I can understand the feel-good factor of rehoming, but if you want a better chance of perhaps 3-5 relatively trouble-free years with younger birds, they will respond to your excellent care and will probably be less stressful for you in the long run.
