The current consensus guesstimate is that it could take 4-5 years and >$1 billion to start US production of the RD-180. This against a 2 to 2.5 year stockpile of engines, and Congress is already talking of embargoing it.
ULA has ~2 yrs worth of engines. Nowhere near the time needed to produce new engines, assuming they could. And assuming they could -at what price? Certainly not less than the Russians. American engines, Aerojet or others - similar situation.
My view is in a few short years SpaceX is going to eat ULAs lunch.
The State Department is also tightening the screws, and SpaceX has filed a powerfully written lawsuit challenging ULA's block buy of 36 Atlas V's. They have a very good case.
As to RD-180 replacements,
The Aerojet Rocketdyne AJ-1E6, which they are already working on as part of a liquid SLS booster upgrade, is an upgrade of their AJ-26. This could be done for about $550 million, about half the cost of tooling up for RD-180.
Then there is Dynetics who has a real fire breathing monster in the works. They plan to resurrect the Saturn V's F1 engine, also for a liquid SLS booster upgrade, only using modern tooling, materials and techniques. The F1C would have nearly 2 million lbf of thrust. They have already reproduced and test fired components of the F1C turbomachinery.