If it is true that someone else wrote it, then it makes sense to me that she would delete the text --I certainly would, not wanting anyone to believe that I wrote it.
But --the question here is if she's telling the truth or if she is lying. Or, if she used very poor judgment and sent someone a text like that, does she realize the mistake and is now committed to you since you are engaged.
I think you need to have a "heart to heart," or "coming to Jesus," or whatever you want to call it conversation where you tell her, "I need to know if I can trust you." Or, "If we marry, I need to know if you are going to be faithful" --something along those lines.
If you think it would be too difficult by yourselves (like if you think she will get too defensive), then I suggest that you do it with a therapist --who can help if she starts to avoid or get defensive, etc.
I would think it would be extremely difficult and not wise to go into a marriage not knowing if you can trust your wife or not.
Please continue to follow up if you need to --