Here's the words off the record - not sure about a few of the lines? The structure sounds like a circle of fifths- i.e. ragtime/vaudeville.
Oh, when the war breaks out in Mexico, I'm a gonna go to Montreal.
I got a 42nd cousin up in Canada, and it's time I paid a friendly ca-a-all.
So I'll pack my grip for the farewell trip, and from the states I'll go-wah-oh.
When the war breaks out in Mexico, I'm a gonna go to Montree—
You'll love it up in Montree—
Climate's nice in Montreal.
Oh, my cousin is a fisherman, like his pa and like his grandpappy.
Oh, if I settle in the states and have me a son, he'll end up a soldier boy like me-wah-he.
So I've changed my tag from Johnny Reb to Jean (Piereally-boo-wah-hoo?).
When the war breaks out in Mexico, I'll be fishin' up in Montree—
Good old peaceful Montree—
Lots of fish in Montreal-aw-al.
Oh, oh, I don't know what we was fightin' about, except it was about too much for me.
But I'll tell you what I was fightin' for, for the last time in my life by gee-wah-hee.
So to keep my health I'm gonna chase myself up to old Montree-he-he-he-he-he-he-he-he.
Unless the war breaks out in Canada, then I'll get some bread and cheese
And, pack up my valise and, head for quiet Mexico--believe me honey--head for quiet Mexico-wo-ho.
If they thought our act was tasty in Atlantic City, they will eat us up in Montreal,