I don't think it would be too hard or take too long - at least, not with war vs. China as the impetus. I'm not saying it would be instantaneous, or that it would go off without bumps or hitches, but I don't think it would be anywhere remotely close to being an insurmountable problem.
The death of American manufacturing capacity has been greatly exaggerated. It is still enormous, and automation has made it more efficient than ever (just ask all the factory workers who have lost their jobs to automation rather than "outsourcing" - the latter outnumber the former). I think what induces a lot of people to think U.S. manufactury is now feeble and pathetic is the fact that the U.S. no longer produces nearly as much in the way of consumer goods as it once did. We used to make most or all of our own clothes, toys, etc. A much greater proportion of our industry involves machining and the like than it once did. China may be able to easily out-produce us when it comes to t-shirts and lawn chairs - but not nearly so much when it comes to tanks and planes and guns and bombs and the like.
I'm not saying American manufacturing is in the greatest shape, but it's better off than it's often made out to be - and you can bet your ass that all the various obstacles the government puts in the way of American industry (which, IMO, account for most of our manufacturing ills) would be dropped toot-sweet for the duration of any openly armed conflict with China - at least with respect to any relevant sectors of production.
