I'd say first meet with the stakeholders and tell them that their presence is really needed (refrain from using the word mandatory) for the health of the project.
If they are still negative or negligible and not attending these important meetings, then I don't see any problem bringing this up with the project sponsor directly.
The best way to bring this up is probably by email, this will first throw responsibility off your shoulder and onto the sponsor's shoulder, and will make this request official and documented. From my experience, executives tend to forward emails to each other unedited, meaning that your name will appear, but if you already talked to them about this issue, then they will definitely know that you are behind "this thing" in any case.
Anyway, the email should be very respectable to the stakeholders: You can say "I understand that Mr. X is so busy currently but I really do appreciate to get some of his time on this project so that we won't have any delays". Again, remember that in 99% of the cases, your email will be forwarded to Mr. X.