Before you get too far, you might want to talk to the Society about your 'options'. Perhaps you may not be allowed to build a wishing well. Perhaps you need to leave it covered. Perhaps it needs to be 'officially designated' as a Historical Landmark. See what I'm saying?
It may turn out that they designate it as a Landmark and simply allow you to cover it over with a steel cap; not a wooden one. It can get ugly. You may need professional, legal advice on this one. For example, did you know that this well was on the site BEFORE you bought it? If not, was it required to be disclosed to you in the Seller's Disclosure Statement? Can the Society prove that the well was part of an old fort? Can you restore the well to its 'original state'? Will you be able to get the 'material' for the well? (Like field stone, and so on ...) Again, it can get ugly.
You may want to 'ask around' to see if there are others in town that have 'historical landmarks' on their property, and see if they were in a similiar situation. YOu can go to your local municipal building and ask to talk to an inspector to see if he knows anyone I'm refering to. This way, you can get some insight as to what you're in for. If you were suppose to be told about the potential historical landmark by the previous owners, and weren't told, you may be able to recover 'damages' from them to either restore the well or at least 'protect yourself from liability' by properly covering the well. Or, get out of the deal all together!
Start asking for insight and consider a good real estate attorney. My best to ya and hope this helps.