We wanted a glass insert with integrated blinds for our back door but since the door is kind of old the only insert we could find that was close is about an inch too wide.

If I remove the old glass how easy would it be to enlarge the current hole so the new glass will fit? I don't want to orde a new door because I think it'd be tough to get the hinges and deadbolt bore to line up and all that.

Metal door ?

There is a place online that will custom make the exact size you need.

Yea it's a metal door