In this usage the phrase "to be or reach" feels awkward to me. I don't think it is technically incorrect -- such a construction using two verbs with the same object is not uncommon. It is, I suppose, a redundancy, because a song can't very well reach number 1 without also being number 1. Thus the second verb does not add any meaning.
Even without the double verb, the first form feels odd.
This song would be difficult to be number 1.
On thinking about it, I realize that in this construction "this song" is the subject of the verb "to be difficult", but it ought to be an indirect object. That is, the song isn't the thing being difficult, it is reaching number 1 that is difficult for the song.
Therefore the form
It would be difficult for this song to reach number 1.
seems to me the best version. It avoids the redundancy, and it gets the subject vs object right.
The song would be the subject in a sentence such as:
This song would be difficult for a baritone to sing..
Here the song is the thing being difficult. I hope that helps make the difference clear.
However, the first version, even if it sounds off, would probably be understood correctly. But it would, I think, feel odd to most native speakers. And I now think it is technically incorrect.