Doesn't Halfmoon Valley, PA date from the 1780's ? If he was living there he might be found in the 1790 PA US census, and in fact, there are a number of John Spencer's living in PA at that time.
If he was at Halfmoon, PA (located in Centre Co., PA which was founded in 1800 from parts of several counties including Huntingdon Co) this may be the 1790 census record for your John Spencer. There is an image of the census record that should be reviewed to see if there were any other Spencer's living in the same area.
"United States Census, 1790," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:XHKJ-832 : accessed 15 October 2020), John Spencer, Huntingdon, Pennsylvania, United States; citing p. 131, NARA microfilm publication M637, (Washington D.C.: National Archives and Records Administration, n.d.), roll 8; FHL microfilm 568,148.
As it turns out there are at least 3 Spencer HOH's found in the 1790 census living in Huntingdon Co., PA. John, James, and Robert. It may be that James or Robert is the father of John ?