Well I guess his barrel isn't threaded.
I don't think the twist rate is going to affect the slugs nearly as much as you think. Any spin is better than no spin and a 2" long section of Rifling at the end is not going to impart very much spin anyway. It will induce some, how much remains to be seen.
Here's a new Rifled Barrel on Ebay for $120US and it appears that they will ship to Canada.
https://www.ebay.com/itm/Mossberg-50...S!-1:rk:5:pf:0
There's a bunch of them on there. All the Smooth Bore Barrels have sights, and all the Rifled Barrels have a Cantelever Scope Mount. They range from $120 to $200 and most all of them are new.
Your gun is an 18" bbl with sights? I'd like that!
Randy