John is listed in the 1890 veteran census living in Coopersdale. He was in Company K, 136th PA Infantry. The Civil War pension index lists John McCurdy of the same unit July 16, 1890 Appl 894.255 Cert 829.548, Widow Catherine July 27, 1905 Appl 832.358 Cert 591.734. You can get his pension from the National Archives in Washington, DC for $75. It's a bit pricey and the genealogical data it may contain can vary considerably, but it could have the very information you are looking for. At the very least it would have information on the widow since she had to prove she was actually married to John. As I understand it the pensions will eventually be online, however, I don't know the timeline. Some are already online at Footnote.com for a fee.
I suggest trying to get John's obituary. It may list the parents or where he was born but it also might list any surviving siblings. The Johnstown Area Genealogical and Historical Society, Cambria County Library or Johnstown Family History Center may be able to help you.