Philip. To answer your question, there's nothing to stop the processing and sensors being integrated on legacy aircraft (other than appetite and money)... but, good luck getting them into the fight for the sensors to be of real practical use; they'll be picked off by Dd-SAMs at range.
Like a 3-legged milking stool, stealth is an essential leg in the capability. Without it the capability collapses. Sensor fusion and networking are the others, as Chip says.
USN uses a different approach that uses a different third leg or perhaps more accurately a fourth leg. Rather than depending on stealth (or just stealth alone), USN is developing active jamming technology, buying dedicated jammer aircraft, and developing tactics to go with them. USN believes there is more than one way to skin the SAM cat and is not putting all their eggs into just stealth.