Replace ECU - but use my Factory Immobilizer and Key/Ignition Tumbler

Hello All - first post.

I've acquired a bit of a tired 99 MT boxster that has a physically damaged ECU in the trunk (plastic cracked/pins bent), Cheap as it doesn't run :-) I've seen lots of threads on immobilizers needing to be replaced and the ECU and key (if I understood correctly) needing to be re-programmed.

My question in this case is whether

1) used ECUs are cheap (down in the 150-200 range), as I assume they are of minimal utility without the immobilizer and key
2) Can I purchase a 99 MT ECU and have it programmed such that I can continue to use my factory key and immobilizer? Or does it only work the other way around (new immobilizer with original ecu)

The ECU would be from a salvage yard so I don't know how to get the porsche "code" that Identifies the box, or can it be retrieved with the PIWIS or other factory tool

Other considerations

- Not worried about the alarm system, door handles, power locks, top or anything else other than the basics of lights, engine, ABS etc as if I can get it running it will be gutted and become a track car or get sold as a track car.
- I have the notarized title and registration to prove ownership to dealer or Indy
- my Indy has both the PIWIS and the new tool, but hasn't done this before - any idea what either tool would call it? He's going to give me the buddy hookup if he can figure it out.

Thanks!