PhilipG,
Even if the jigs etc are all available, there will be a lot of work involved in redesigning the IT systems on the F22 as the original processors etc are no longer available, yes the USAF has a stock of them for repairs etc to the present fleet.
They did indeed make a lifetime buy a long time ago. I can remember reading about it at the time, marvelling how they were having to make such a purchase before the aircraft had properly entered service!
What Is a Lifetime Buy?
As to what a 'lifetime' buy of CPUs actually is is debatable. They would have purchased according to what was then their best guess at MTBF, mishap rates, etc, and then doubled or tripled that. Now a good few years later they'll be in a good position to assess their actual spares usage. Unless there's been a serious cock up I'd bet they've enough to support re-opening the line.
Use an Upgrade Program to Liberate Old Spares
Even if the spares pile isn't quite as big as all that they could still reopen the line and simultaneously kick-off an avionics update program for the whole fleet.
That way they could build and fly a new batch having pinched what they have in stock, and use the upgrade program to completely replace the diminished spares stockpile. That means running the spares pile a bit thin, but that is offset by the situation hopefully being short term.
Also as the upgrade rolls out they can put old systems from upgraded aircraft back into store. Of course, if the upgrade development program goes wrong they could have a serious shortfall!
Make the Chips Again
Yet another option is to go along to the semicon fabs who specialise in re-manufacturing old parts. It's actually quite cheap (like under $1million easy). These fabs have bought the masks for old designs and stored them, and can easily put them back into limited production should anyone ask. They can do it because whilst a 5 year old fab is no longer state of the art and so can't make modern designs, it is easily capable of making an i960 from the 1990s. Of course that assumes that the masks for all the important chips are still in existence; Intel might have been a bit more possessive of them than other manufacturers and not sold them on. I've heard this approach has been taken by some other defence equipment programs.
Just Port It Anyway!
I believe back in the early days (mid 1990s?) of Eurofighter they had the same problem, and
AFAIK they did port the software to a newer CPU. Provided the operating system environment is the same there's no particular risk, just a bunch of testing. Things only get really nasty when the operating system has gone obsolete too and isn't available on a more modern CPU. Then you have to change the source code itself, and that can be very painful.
It'll be interesting to see if they do decide to restart production, and the manner in which they do it.