Hi Sue
Always hard with illegitimate births isn't it to be certain on father's details

You are fortunate though that father Malcolm accepted paternity and registered birth which has given you a little bit of extra info to try to work with.
Having said that, I'm not seeing many potential candidates for him. There is this entry in 1871, not sure if you have seen it. One big problem though, as this Malcolm shows as married as at 1871:
Janet Martland 56, head
Mary Forrester 30, daughter
Janet Forrester 11, granddaughter
Maggie Forrester 9 granddaughter
Mary Forrester 6 granddaughter
Malcom Mcgougan 30, son in law, fisherman b. Campbeltown
Flora Mcgougan 28 daughter
Malcom Mcgougan 2 grandson
Address: High Street, Campbeltown
Malcolm McGougan and Flora Maitland married on 11 MAY 1869 in Campbeltown and I can see them in the censuses up to 1901, with Malcolm still showing as a fisherman. From what you have, Malcolm, the fisherman/farm servant showed as deceased by the time of son John's marriage in 1892.
Have you found any possible candidates for John's father?
Monica