your best bet would be to get a euro owners manual...
and its not really similair to a new wrx engine, so getting parts will require research or a good hook-up. The oil filter may be similair, but that along with maintanance will be the least of your problems. Keeping it running smooth on our crappola gas without a major overhaul will be the biggest step to overcoming.
If you don't want to floor it, you'd most likely be ok. But otherwise, your running a risk by not running it on race-gas.
Either way you look at it, you'd be paying a large chunk for engine work(to run on our gas(untested)) or continue paying for very high octane fuel, and either one will cost a lot in the long run.
If I were you I'd give it a few months until it had been done by other people... then your not the test dummy...
But you don't have to take my word for it,