If everything's working when assembled, reattach the display by following the iMac Intel 27" Retina 5K Display Adhesive Strip Replacement guide.

Using the adhesive standoffs, attach the board with buttons on it to the back of the iMac case so you can use it when necessary.

Power it up do computing.

If you want to mount your old/new monitor there are VESA mounting solutions like this one available.