The 1841 census transcription at
https://www.freecen.org.uk/cgi/search.pl lists at Clune, Deskford Ewan McKay, 50; Margaret McKay, 40; William McKay, 15; Margaret McKay, 9; Peter McKay, 7; Joseph McKay, 3, all born in Banffshire except Ewan.
In 1851, at Allan's Lane, Parish of Fordyce (I think this is in the village of Fordyce but since the transcription helpfully omits this detail I can't be certain without looking at the original) are John Russel, 53; his mother Janet Russel, 84; his sister Margaret McKay, 51; his nephews William, 25; Peter, 16; Joseph, 14; and niece Jane, 10.
You can view the full transcription at FreeCEN, and you should look at the originals at
www.scotlandspeople.gov.uk to make sure there are no transcription errors.
So it looks very much as if the Ewan McKay who died in 1842 is the father of this family. (Where did you find his date of death, by the way?)
There are 289 M(a)ckays in Banffshire in the 1841 census, and I ran an eye down the list (on FreeCEN) to see if there are any clues but nothing leapt off the page.
Adults' ages in the 1841 census were supposed to be rounded down to the nearest 5 years, and the census was taken on 7 June, so if Ewan's age is accurate he was born between 8 June 1786 and 7 June 1791.
He named his eldest son John, and his second daughter was Anne, so there is a slightly raised probability that his parents were John and Anne. However this is not certain, and the water is further muddied if, as I suspect, Margaret's father was also John. (Was she the daughter of John Russell and Janet Lawrence?)
The Aberdeen and North-East Scotland Family History Society's online index of gravestone inscriptions does not include the old kirkyard at Fordyce, which is odd because I thought that they had published a booklet on Fordyce. However no such booklet appears in their list of publications. Perhaps it is out of print.
There is a published book listing the transcriptions of all stones in Fordyce Old Kirkyard, recorded by W Cramond in 1886, but I do not have a copy. However see
https://www.ancestry.co.uk/boards/thread.aspx?o=0&m=4847.1.1.1.2.1.1&p=localities.britisles.scotland.ban.general.
However most people did not have gravestones, and as a farm labourer with a young family, he probably doesn't have one.
There are no records of the baptism of a Ewan or Ewen McKay or Mackay between 1780 and 1795 at Scotland's People, and the chances of a missing register lurking somewhere are remote.
Have you looked at all the baptisms of his children on Scotland's People, to see if the names of witnesses are listed? If, for example, a certain M(a)ckay was a witness to more than one of them, could it be Ewan's brother, and could he have survived until the start of civil registration in 1855?
See
http://www.geograph.org.uk/gridref/NJ5765