Elliott Investment Management is in talks to buy a stake in a package of infrastructure assets, including a pipeline that carries Russian natural gas to Europe—a deal that could form a template for reviving Moscow’s once-mighty energy business by involving American investors. The U.S. hedge fund, headed by billionaire founder Paul Singer, is considering a stake in the Bulgarian extension of the TurkStream pipeline, along with access to a network of data centers, data cables and other infrastructure assets, according to people familiar with the matter.