If John Ferguson was a farmer he will likely have lived in the same area most of his life and he’ll likely have married locally. I looked in the 1831 census of Co. Londonderry to see where the name Garven/Garvin was found. It’s in 5 parishes: Drumachose, Tamlaght Finlagan, Ballyscullion, Macosquin & Desertoghil. I then searched the name John/Jno Ferguson to see if there were any in those parishes. Only Drumachose has both Garven & John Ferguson. There were 2 households both in the townland of Rathbrady Beg which is the town of Limavady. One was Presbyterian (3 males & 3 females) and the other contained 2 families, both Church of Ireland (4 males and 4 females in both).
http://www.census.nationalarchives.ie/search/
Now the weakness of this approach is that it assumes John was head of his own household in 1831. If he was unmarried and living with his parents then the head of household is probably not called John. But I am sure you get the idea; if you compare all the parishes with any Ferguson and any Garven, and eliminate Fergusons who weren’t Presbyterian you’ll end up with a fairly short list.
If John Ferguson was a farmer then you wouldn’t expect him to be living in a town so I am not sure that the Rathbrady Beg household is yours but you can use that search principle to identify likely parishes where the families may have lived.
Few Presbyterian records are on-line. However most have been copied and are in PRONI (the public record office) in Belfast. There are 4 Presbyterian churches in Drumachose (the area around Limavady). Derramore’s records start 1825, Drumachose in 1827, 1st Limavady 1832 (with gaps) & 2nd Limavady 1845. A personal visit is required to access them. Access to the records there is free. This link explains what records exist, parish by parish:
https://www.nidirect.gov.uk/publications/proni-guide-church-records
If you are unable to go yourself, you could employ a researcher. Researchers in the PRONI area: http://sgni.net
The main shipping agent in Londonderry in the 1840s and 1850s was J & J Cook. They had regular sailings to Philadelphia, St John (New Brunswick) & Quebec. Their passenger records have survived and those for Feb 1847 – 1849 are on-line on various sites including PRONI (ref D2892/1/1). I searched them and did not see a John, Nancy or George Ferguson listed, though there was a William Ferguson, from Limavady of all places.