I am also afflicted with an ej18 NA in a 96 imp. If I was going for an engine swap, I would seek out (for all the effort and expense involved) an ej20 or ej22. Seeing as how a wiring harness swap is imminent, you may as well have that ej20 in there, reliably, as it is much more at home and has a huge aftermarket and user knowledge behind it. And our little imprezas are smaller and lighter that those bugeye guys too. Here in the Reno tahoe area, ive not seen anyone have a very long shelf life with one of those type turbo subys either. I sure have passed a plenty of them with my 150k ej18 NA going up winding old highway 40. Of course, it aint cheap neither, but engne swap arent. I myself am just rolling my pennies, canning and bottling, and just waiting until i finish nursing school and get to work to create my monster, the world will then be sorry.
Until then ive concentrated on the stuff I can piece together with smaller funds: suspension from cooler models, brake swap, 86-ing those 14x5.5's, and other universal stuff.
Another option thats cool but maybe less reliable is the EJ18t do it yourselfer, do a search, i found some guys doing obscene things to ej18s with some seeming success, as they tout the block as pretty darn strong, and there seems to be no wiring harness mess.
Underdog Arkies unite!