Let's see. You got Mrs. Vorhees killing counselors, and the Mystery Inc. comes to solve the mystery. Hmmm....Mystery Inc may be prone to run for police help upon seeing the first dead victim, since their mysteries usually ran the way of stolen objects or monsters scaring people off. However, given that they may not be able to leave the camp, since the Mystery Inc. group never really splits up, Mrs. Vorhees may have trouble killing them. Remember, Vorhees used her familarity with the couselors to get close to her victims, and know where they may become alone and split from the group, and gets them unaware, like when they were having sex. Since Fred always traveled with Daphne and/or Velma, and Shaggy was always with Scooby and sometimes Velma, and since the Mystery Inc kids didn't have sex with each other, and don't really trust anyone (everyone is a suspect with them, espcially with murder afoot), their guard isn't let down as were the couselors. Without that advantage, and given the fact that Vorhees didn't dress up like a monster, Scooby may actually attack her in order to save his friends from death. Also, Fred could deck her and knock her ass out. Hell, Shaggy may actually deck her if he saw his friends in danger.
Bottom line, Mrs. Vorhees gets captured and taken away by the police, and Jason never sees his mother be decapitated.