Rebuilds can be very expensive. For an EJ25 I doubt it would be worth it, especially considering you've got so many miles on that one. 270k miles means you probably have to bore the cylinder (which could mean new pistons), clean the valves, regrind the valve seats, etc, etc, etc. For just machining, you're probably looking at $1k. Then there's seals, gaskets, etc. Of course, your engine would be practically new after that but it'll be way, way less expensive to just get a used engine like Pat suggested. That's my vote anyhow.
JD