^This. BTDT. Good used WRX shortblocks are like hen's teeth...and now you understand why.
Go with a fresh factory one, have somebody who knows the drill do the swap-out with your blown motor. Make sure they clean the heads super well, that blown motor is full of debris washed everywhere thru oil passages. If short block does not have oil pump, replace that as well, it's wrecked due to pumping fluid full of metal chunks.
Condolences.