Scan the photo and play with the contrasts if B&W the hues if color in Photoshop or something like that.

I will warn you that even if you do get some of the names unless you have possible cemeteries, the names might not help unless it is in an area where all the cemetery markers are in a database. Even then unless the names are unusual, they might not help.

Stick with the stone you are interested with. If some one took the photo they must have thought it was readable or they would not have kept it. Concentrat on the 100 to 1 vs the million to 1 shot.