All Su aru engines have identical bolt pattern even BRZ ever since EJ engons laucnhed in early 90s. Although brz has 1 difference being starter motor being mounted on other side to all other subaru cars.
You could easily mount up transmission from 6speed XV, Jmpreza, forester or usdm va WRX. It ideally should have a fottong rear diff aswell.
Problem is wiring, CANBUD and fact 1.6turbo was never sold with a manual. Perhaps ecu from 1.6i XV can work with some modifications. It might need guage cluster from XV aswell.