Looks like an Imperial or similar to me but I'm not an eggspert either. I believe the firebox is supposed to come out. It looks to be intact, not cracked or anything, if it were me I might be inclined just to throw a grate in there and fire it up, I'm not sure I'd want to force it out for fear of the firebox cracking.
Maybe a little grease and a couple cooks might loosen it up even.
It has a sliding vent already, grab a BGE cast iron lid if it didn't come with one and let er rip.