If Ney drops out in-between 79 and 76 days before the primary, the Republican party can essentially pick his replacement as long as it is done before the 76th day before the elections.
After 76 days before the election, the law is not clear, though there is nothing in the Code forbidding a candidate from withdrawing, however there is no mechanism for the party to nominate a candidate for the seat. Chances are that it would go to the courts.
"If Ney drops out in-between 79 and 76 days before the primary, the Republican party can essentially pick his replacement as long as it is done before the 76th day before the elections."