Okay, folks, back to Eusebius's Ecclesiastical History, specifically 2.13-14. Here we see the report that Simon Magus came to Rome during the reign of Claudius and there he contended with Simon Peter. This story can in fact be broken down into three distinct claims: that Magus (as I will call him henceforth to easily distinguish him from Peter) went to Rome during Claudius's reign, that Peter went to Rome during Claudius's reign, that there they interacted. Let us consider the first claim in this post.
The report that Magus went to Rome undoubtedly dates back to the mid-second century at the latest. It is found in Justin Martyr's First Apology 26, which in fact Eusebius cites as his source on this matter. It should be noted that there is nothing particularly unbelievable about the account. It was not at all uncommon for "oriental" teachers to travel to Rome. If Magus was active in Samaria in the 30s, as per Acts 8, then it's not at all inconceivable that he was active in Rome sometime in the 40s through mid-50s. Yet I have reason to pause, and the reason that I have to pause has to do with something else that Justin says, namely that there was a statute devoted to Magus standing on the river Tiber, bearing the inscription Simoni Deo Sancto, "To Simon, the holy god."
The interesting thing about this statue is that it's likely been discovered. In 1574 a statue was found on Tiber island, pretty much where Justin said it should be, bearing the inscription Semoni Sanco Deo. Now, I'm hardly an expert in Roman religion or archaeology, but from what I have read the general consensus is that this is a reference to the god Semo Sancus. Now, the coincidence of place and inscription would seem to make it likely that this is the statue to which Justin is referring. He has apparently misread the inscription as a reference to Simon Magus. This raises the possibility that Justin inferred from this statue that Magus came to Rome.
Frankly though that seems a bit of a leap for Justin to have made. If all he knew about Magus came from Acts 8 it is not clear to me why he would associate Magus with the statue and create for him a journey to Rome just to account for the statue's existence. It seems to me much easier to account for Justin's misinterpretation of the inscription to Semo Sancus if he already had reason to think that Magus had spent time in Rome. From whence would he have gotten such a reason? One suspects that it might have something to do with the fact that Justin is himself from Samaria. As such, it seems likely that when he tells us in Apology 26 that all the Samaritans worship Magus he is aware of a Magus-cult in his homeland. It is not at all inconceivable then that he had access to no longer extant traditions (oral or written) about Magus, and that these traditions referred to a journey to Rome. Indeed, absent such accounts I have a hard time accounting for how he came to believe that the statue referred to Magus. It seems moreover likely that the source(s) in question dated Magus's visit to Claudius's reign: there is no mention of Peter in Apology 26, which means that Justin probably is not drawing inferences about the visit from any material that puts Peter in Rome during that reign. This also raises interesting questions about whether later accounts about the Magus and Peter in Rome, such as those found in the Acts of Peter and the Pseudo-Clementine literature, might also draw in part upon such non-Christian sources.
Now, whether or not Magus came to Rome still remains an open question, but I do think it likely that this tradition predates Justin and comes from non-Christian sources. Combined with the fact that such a journey is not at all implausible I am inclined to provisionally affirm that Magus did in fact travel to Rome during Claudius's reign.