Actually China was offered the PAK FA co-dev first, but then we pass on it.
Now it's possible that we hit problems and we are in a worse situation than we thought, but at this point, I don't think the Russians will just sell it to us, especially since the Russian engines have problems too.
Even American engines are not perfect at this particular point and needs work, and the Americans has had the F-22 since 06.
So this story makes no sense, not only would we be unlikely to buy, but the Russians are unlikely to have an engine to sell.
China was so confident in its ability to produce a fifth generation fighter by itself that it turned down an offer from Russia for joint development.
In a worse-case scenario, the first J-20s will be powered by more powerful versions of the WS-10A engine.