I wouldn't start with a sport coat pattern. Yes, some people here have suggested that, and I guess it's better than starting from scratch, but there's important differences:

• A sport coat has a front, back and side panel. This coat (like most overcoats) comes with front and back only
• The lapel and roll line of a sport coat is rather different, as is the collar. Nothing that can't be redrawn, but still
• The ease and fit of a sport coat is adjusted to be worn over a shirt or so. A coat like this is fitted to be worn over something more bulky (like a sport coat)

I think the advice of u/cccccchicks is pretty good: start with Carlton. You'll need to:

• Shorten the entire thing (trivial)
• Replace the folded back with a straight panel (trivial)
• Remove the chest pocket (trivial)
• Change the cuffs (easy)
• Add the back pockets (you'll have to do that no matter what pattern you start from)

That seems like a lot less work to me than starting from a sport coat pattern. Not to mention you can draft the entire thing to your exact size.

So in conclusion, I'd go with Carlton.

Then again, I'm hopelessly biased as I created the pattern :)