The key transponder (looks like a pill inside the key fob) + ECU + immobilizer are all programmed at the factory with the same security code. If one of those three components does not communicate with the others OR if the security code isn't the same, the ECU will not allow the engine to operate (the engine won't even turn over).
The bottom line is that there is no way to work around the immobilizer.
I'll say it again so we're perfectly clear: without the proper signal from the immobilizer and key transponder, the ECU will not start the engine. Period.
Why did Porsche design it this way? To stop car thieves from doing exactly what you want to do (remove the immobilizer and still be able to operate the car). Now, I'm not saying that you're a car thief, but if it was easy to work around the theft prevention system, then it wouldn't be a very good theft prevention system.
The easiest solution is to buy a replacement immobilizer and have it programmed to match the key transponder and ECU. Then install all three components in whatever car you like and the engine should run fine. Another option is to buy a replacement matched set (key/immob/ECU). Another option is to use a non-stock ECU (then you won't need the key transponder or immobilizer) but you'll have to pay someone to program it on a dyno. Another option is to find someone who has hacked the ECU to disable the immobilizer function (search under "boxster car thieves"
).