Take it from me as I've had some knob use a wrx ecu on my forester many moons ago and it drove find, no warnings. But as i was about to get a tune on the dyno, the AFRs were all over the shop and it went bang, Was a ticking time bomb.

The issue is not with the condition of the engine (it may be a contributing factor, if it wasn't looked after properly or abused and unless you did a full compression check of the original engine, you wont know).

You are using a Forester ECU, which is designed to run a higher compression Forester engine. So the ignition, timing, etc is not suited to a WRX (or STi) motor which has a lower compression and hence different ignition, timing, etc. Short term its fine, but all the safety built into your Forester ECU is there to protect a Forester motor. Easiest way to check condition now is put it on a dyno and get the AFR, again, wideband not the AFR reading that you get from your run of the mill $60 gauge. That's stoichiometric AFR, means shit all.


Feeding it U98 will buy you some time/insurance but does not eliminate the fact that its a ticking time bomb

Again, I've been through this. Not worth spending twice on a new engine.
Its a friendly advise here