With a 97 wrx you can get the sports pack option with ESL which gives antilag + LC. Getting the extra chip installed costs about £30 for ESL to mod your existing ECU by soldering on the additional board so you either send your own or source a second standard ECU and send them that keeping your own on the road whilst you await the turnaround.
I dont know of ECU's from other Subarus that would fit as a straight plug and play - as I understand it the 00/96 differs to 97/98 and they differ to 99/00 - all differ to newage
But the one point I would make is going aftermarket gives you flexibility so factor in some time with someone who can setup the ECU specifically for your car, either on the road or book a dyno session.
Whilst at the ESL course the concept of the ECU 'learning' and adjusting based on wideband inputs like on newer ECU's was discussed but it is not a function currently in place so plugging in someone elses which has been set to their car means it would potentially need adjusting to suit your motor