A 1991 Eclipse GS for $1,600 with 170,000 miles is headed for the junkyard. Don't even bother. You can find a 91 Eclipse GSX with 50,000 less miles for around $1,000.

But to answer your question, a 170,000 mile 6 bolt NA 4G63 will last plenty long as long as it is properly maintained. I've seen a couple 200,000 mile cars running around just fine. There was a 90 GSX with 230,000 or so miles running 12's at the shootout last year.