You could put in an SR20 if you think going from 3.0 to 2.0 is an upgrade.
ease up a bit guy.... the JDM craze has gotten the best of you. Swapping an engine doesn't always mean it's better, much less the best expenditure of your money.
Your engine will hold over 400hp. Now, would you rather put about $1500 into making it get there, or pay $2k to do an "engine swap" which would possibly even have a net hp under what you already have?
Now...as for what can you put in.. anything you want. If you have the wallet, there's a way. There's been 350's, and more put in these cars.
And don't remember that while one guy is obsessed with being the first to do something, there's millions that are smart enough to know very well why nobody's done it before.