Rand Corporation did a paper in 2010 on the cost to restart the F-22 in the future:
http://www.rand.org/content/dam/rand..._TR831.sum.pdf
It would seem an improvement item for the F-22, if it were to be restarted, is the stealth cladding that has been used. I believe it contributes the most to overall maintenance costs for the aircraft. Perhaps that used on the F-35 would be better.