Are most Mexican silver pesos - replicas?

No, the majority of them are genuine coins. Until 1967 the Mexican peso was partially silver (though, in 1967 it was only 10% silver) and all the peso coins dated 1967 and earlier contain some silver. Since a lot of them were made, most are not replicas and they are genuine coins. Since many of them were made and they aren't considered to be very attractive and rare coins, they are usually sold for little more than their silver content. Though there are a few replica pesos, the vast majority of them are genuine.