Update: I'm incompetent. The engine controller was just disconnected, but nested away in its original position. Plugged everything back in, and it starts... just sounds like a bad motorcycle.

But perhaps the bits of information below might be useful for anyone who actually had their ECU stolen / needs it replaced. Though because nothing actually needed replacement, you don't get a conclusion to this saga or to whether any of this would have actually worked 🤪.



So the ECU on my 99 Accord LX was stolen (along with other things, like the catalytic converter, but that's out of scope for any DIY job), and I'm looking into inexpensive options for repair.
It has a F23A1 engine.

So first off, because the original is gone, I don't have a reference part number. There are a lot of 37820-PAA-L61 on eBay from 99 Accords - will these be compatible? (also if it matters, this is California, so I'd like to know if there are smog implications)
I did call the dealership about replacement parts, and they mentioned 37820-PAA-407 without needing my VIN, so it seems like there may be a range of acceptable replacement parts. It seems (speculation) that PAA is the important part, and 407 / L61 is just a version number or something?
(also, for the curious, the -407 is a $2.3k part from the dealership, which along with a replacement catalytic converter would total the car)

Second would be programming a replacement ECU.
One local locksmith would be able to do the immobilizer programming, but couldn't pair the ECU with the antenna ring. I haven't heard anything about the ECU and antenna ring pairing anywhere else, though "plug-and-play" ECUs on eBay are sold with an antenna ring and key.
Another locksmith wasn't sure, but could come out and try.
The dealership wants ~$200 for the programming service, and would need the car taken there - which is a bit difficult with a car that doesn't run and may have other damage that isn't obvious yet.

That being said, if possible I'd rather spend the money on a scan tool to do it myself, instead of paying for service.
From this DIY video of ECU replacement reprogramming, it seems like there are two steps: rewrite the VIN, then do a replacement PCM operation on the immobilizer. (presumably one of these steps also pairs the antenna ring, if that is a thing at all?)
The video description suggests it's using a Launch X431 of some kind (expensive), for which a modern successor (is it?) seems to be Thinkdiag (less expensive, <$100). However, web documentation for this tool is ... lacking and disorganized, to put it nicely. The vendor also claims that there is no immobilizer coverage for models before 2003, though I'm not super confident in the accuracy of that statement, then walked back on that after further questions.
So, does anyone have recommendations for scan tools that cost similar to dealership or locksmith service that could rewrite the ECU VIN and reset the immobilizer? Or is this going to be cost-ineffective for a one time repair job?

I'm also happy to take suggestions for other approaches to replacing the ECU (or dealing with the aftermath of a catalytic converter theft in general).
I know that immobilizer bypass chips exist, though I'm reluctant to open up the ECU, desolder (or otherwise remove, perhaps destructively) a daughterboard, and place a new one in. And as ineffective as the immobilizer was in preventing theft, it still seems (very marginally?) better than nothing?