That is a gorgeous saddle!
It does look a bit wide for the horse, but I wouldn't try to alter it. It would only ruin it to do so. I would keep it and hope it fits eventually (a nice 1" thick pad would help) or keep it and hope you have a horse it fits in the future even if you have to buy a different saddle for this particular horse.
But it's a gorgeous saddle and worth keeping even if it doesn't fit this particular horse at this particular moment. That's my .02 cents.