Welcome to the forums! And hope you are enjoying that Montana.
Considering yours is a 2019 model, it's still pretty new. The easiest way to get a replacement door is to go back to your purchasing dealership and see if they can order one for you. That is really the easiest way to do it, but not necessarily the cheapest way to go. But it is the sure way to go.
As far as repairing the original door, that will involve some carpentry skills, a little imagination, some time, space, and a little money to repair yourself. If it were mine, I'd find a way to repair the door, but I'm also a non-professional carpenter too (Meaning I don't do carpentry work for a living or am bonded). But, I've done my share of remodeling several houses with a hammer in my hand since I was 6 years old too (and now I'm 66).
If you still want to replace instead of fix, you also might get lucky at some RV salvage places too. Depending upon where you live, you might have one nearby you. Check the internet.
Another thing, you might check
here too for more specifics about your Montana. I have been on both the Keystone and the Montana forums for some time now, having owned nothing but Keystone campers. The Keystone site (this site) is great for anything related to Keystone and you'll find information here you won't find of the Montana site. On the Montana site, you'll find specific things unique to Montana's only. Between the 2 sites, you'll find a LOT of information and LOTS of opportunities to toss questions out "there" a lot of very knowledgeable, very informed, and very experienced RVers!
Welcome!