Replace them with the MLS units. I can’t remember the part numbers but others will. They’re from the EJ25 STi WRX model.
You’ll thank yourself for going to that effort later. If you don’t, they’ll end up doing the external coolant leak thing.
If money is tight, drop it in and run it. Over time you’ll see the leak develop. It won’t damage anything, especially if you regularly check the coolant levels.
Cheers
Bennie