Cool problem. Here’s my take.
You bought the contemporary for a reason... seemingly to drive one while the other was in progress.
So if it were me, I’d leave it (the contemporary) drivable for as long as it took for me to finish the original semi-scratch build.
Then of course, once that’s done, I’d move my attention to fixing the contemporary and sliding in another FE.
It’s a WIN WIN all round!
Enjoy.