What parts of my syllabus or course materials are vital to retain in the online course? 

Don’t be afraid to keep it simple.  Even though the semester is already underway, it may help to back way up and look at your course objectives. While you probably have some great face-to-face activities planned, they may not translate well to an online course. Our goal is not to somehow figure out how to replicate our activities but instead to accomplish our course objectives. Don’t try to include all the bells and whistles. Use tools that you are familiar to you already, or that will minimize the learning curve for you and your students. Start small and basic. You can revise later if you have the opportunity to do so.  Again, this is a place where brainstorming with someone else may help. If needed, call a colleague or see the contacts in #1.