I have had many pumpkins once you have a problem with an internal split that creates just the smallest opening the pumpkin is basically history. I have tried fungucides grafting wax silicon etc. At that point the pumpkin is not sound and will rot soon. My first response is how can I save my prized baby. There probably are some sneaky solutions. They borderline on cheating . My recommendation would be face the music and walk away. Brock