You may not want to hear this, but I personally think the all brick fireplace in House 2 is head-and-shoulders better than the one in House 1. Especially the way it ties in so nicely with the deeply-coved ceiling. It has balance, substance and craftsmanship, things that House 1's fireplace is completely lacking--to me, House 1 looks like a fake fireplace insert, slapped together by someone in a hurry, and looking almost "imitation." And there's also the added benefit of some solar gain from House 2's brick side facing the sunroom, releasing its stored heat at night when you'd normally turn the furnace down.
But to answer your question, if you insist on tearing out the beautiful piece of workmanship, it should be a fairly straight-forward operation. Plan on sistering in some floor and ceiling framing to support the patched floor and ceiling covering (the flue and block assembly probably "necks down" in the attic, so you shouldn't have any roof penetration problems). And perfectly matching what's there will be the biggest challenge, to avoid having things permanently look like an "add-on."