Would the house you bought have cost less without this secondary rental? If so, then it's not really $0, whatever the appraisal says. On the other hand, if you would have bought the main house at the price you paid even without the rental, I suppose it might as well be $0.
Without putting a price on what you paid, the rental more than meets the 1% rule whether you rehab or not. I'd be looking at the following things -
1. What work is needed to keep the rental in sound structural condition? This is mostly about making sure that plumbing and electrics are in sound condition inside and the outside has a sound roof, clean and functioning gutters, paint or other protective coating on windows and outside walls as necessary and that wildlife and plantlife is not invading. As a minimum you need to do all this in any case.
2. What do you think of the current tenant? Do you want them to stay? Are you prepared to give them notice, or to just keep upping the rent until they leave of their own accord? Would they move out for renovations and then move back at the higher rent?
3. You have a historic home in a historic neighbourhood. To what extent are you proposing to renovate what is there or to replace/update? Are there any legal restrictions on what you can do? Would updating destroy current charm and value, or look out of place?
It sounds to me like a nice problem to have, on the whole.