perhaps a scout round Rathven and surrounding cemeteries.
The extant stones in Rathven/Buckie/Enzie were recorded by the Council in 1978-1979 and the results are indexed in LIBINDX libindx.moray.gov.uk/mainmenu.asp. I did look there but didn't find anything on Peter and Isabel, so barring transcription errors there isn't a gravestone.
Plan a day or two in Edinburgh before you get here for the legwork.
It is possible that there might be a mortcloth record in the Rathven kirk session records. These are held by the National Records of Scotland. They have been digitised but are not (yet) available on Scotland's People. You can view them in the Historical Search Room in General Register House in Edinburgh, or in the Aberdeen City Archives.
A lot of the land in Rathven was part of Seafield Estates, and indeed the online 1865 Valuation Roll shows Knowhead of Findochty as belonging to the Earl of Seafield. There could well be some rental records in the Seafield Estate archives. These too are in the Historical Search Room in Edinburgh. (I found in the Seafield Estate archives an actual letter written in 1771 by my several-greats-grandfather offering to rent a farm from Seafield Estates in Rothes, and rental records than enabled me to pin down to within a year when my ancestors took on or gave up the tenancies of farms around there.)
Take a good look at the NRS web site and catalogues before you go there, as it takes a while to get familiar with the cataloguing and indexing system, and also some documents may have to brought in from external storage and need to be ordered in advance. See
https://www.nrscotland.gov.uk/One thought that might be worth bearing in mind is that there were a lot of Roman Catholics in Rathven, and specifically in the western part of the parish, known as Enzie (pronounce it "ing-y") and they included some Copland families. I still have some distant Copland relatives who are RCs, and there was a Monsignor John Copland (1920-2002) who was incumbent of the RC church in Keith.