You should also look at Subaru conversions.
I'm not a lover of Subi but those are the most logical upgrade IMO.
Both big 6 and turbo versions are doable, and a super simple NA 4 will make a quick dependable 914...
Another possibility is the Honda K series.
A recent thread regarding this:
http://www.914world.com/bbs2/index.php?showtopic=372943A k20 will be nearly 1" shorter than that K24 and with a turbo, 500 hp is not that hard to hit.
That thread shows commercially available transmission adaption to stick a Honda K on a Boxster transmission.
Engine to trans adaption is the HARDEST thing to overcome when sticking one OEM onto another.
I'd like to install a 3.6l onto a Boxster 6 speed. Mine would be a GM LGX (huge torque curve/plateau?).
Nobody makes adaption and that makes my idea MUCH more difficult.
If you're not stuck on a Porsche engine the Germans tend to re-use things like bell housing patterns and there is a whole SLEW of German (Audi/VW/Bentley) stuff that will bolt up. Even the twin turbo W12 Bentley has a chance...

Best guy to ask about this is Clay Perrine.
Or you can put a Ferrari engine in it .