You mentioned a couple options here. You could rebuild and then turbo the current motor or swap for an h22a and keep it NA. If you did the rebuild, the mileage won't really matter much. BUT if you decide not to do the rebuild, or that you can't afford it, I'd say don't waste your time and money on a turbo setup for a motor that old with that many miles on it. Instead I'd do the swap. But if you can afford it, the rebuilt h23 with a turbo would have the potential to outpower an NA h22a by a long shot. You could definately go with a custom turbo setup as well. Also, I don't know if a T4 would be the best option, but that's definately something that can be decided later on down the road, when you decide what you're going to do and how much power you plan on making. If you haven't noticed by now, most of this depends completely on how much money you plan on dumping into it
