This is mildly embarrassing. I actually have a copy of "Hogg's Jacobite Relics", which I'd nearly forgotten about. Hogg does indeed have "scour the DURK" as the lyric. [ Page 97, Second edition ]
Alan Reid sings "durk" on the "Battlefield Band" recording.
I agree with Bob Knight's theory above, but although "dirk" is almost certainly meant, and is perfectly logical, the word as printed in "Hogg's" is spelt "durk"and the singers I've heard sing the song seem to have kept to that pronunciation.