Most 671's have smaller transmissions and of course the mounts will need to be re done. The cost of all that work will not be cheap unless you hire illegals.
I would guess ( I have done repowers before but not 12v71's ) it will cost every bit of 12K just to get the old ones out and the new ones in. Not including cranes, Glass work and such. $3-5K or more
Say the 2 671's cost $10K each. Now the $12K to do the swap. That's more than a rebuild and you no longer have the ability to plane. Even if you get 10 for the RTO engine I don't see the benefit.