At St Albans South Signal Box we've a number of cast iron signs, mostly also Midland Railway. Those being displayed outside are cleaned and repainted, but we have several we're keeping indoors which we're leaving in their 'as received' state for comparison. But that's 'museum' practice.
I understand that there are two ways that memorabilia collectors look at things. Some prefer to restore, as you plan to do, some prefer to leave in the 'as is' state in the belief that that's not interfering with its original condition.
There is a photograph of an identical sign to yours in "Midland Style" by the Historic Model Railway Society, published (2nd edition) in 1978. The accompanying text says that the background was dark blue with white letters, so it may not be black Hammerite you need! By the way, clean and paint the back as well - the MR sometimes didn't do that and the sign will deteriorate even more if it's displayed outside without the back being protected. (And note that £10 in 1893 could be rather more than a month's wages!)