Your hen almost certainly has Marek's. She may not be showing symptoms, but she has it. Good/Bad news--so do about 80% of the birds in the United States, and very few die of Marek's.
I would get an older hen or two to keep your girl company--older hens are more likely to be able to keep their place in the pecking order, without too much bullying and stress. Chicks are going to have to be separated until they're old enough to fend for themselves with her--and she'll be alone for all of that time.
If you get older hens, they'll almost always already
have Marek's, because it's such a prevalent disease. Because of this, and because the vaccination is quite often ineffective in older birds, I would not bother vaccinating them.
If you do decide to get chicks, let me start by saying that I have Marek's in my flock (most people do) and have raised many chicks with the flock. Only one chick ever got Marek's, and out of a few hundred total birds, I've only ever had two cases of Marek's, without vaccinations.
The vaccine has been shown to be about 90% effective. So if it would give you peace of mind, please, by all means--vaccinate any new chicks.
End Note: I personally would not sell or buy chickens from a flock vaccinated for Marek's because the vaccine is what is known as a "leaky vaccine"--the vaccinated birds can still get Marek's, but they keep the symptoms masked. That means that birds from a vaccinated flock can carry a really bad strain of Marek's, without ever showing signs--and could infect my flock with a killing strain of Marek's, worse than the one that they already have.
Check out this
Marek's Article if you want to know more about Marek's.
