The 210's were made for a number of years. There are four different parts books that cover all the changes to the 210's. I've given you a link below that will take you to Ingersoll's website and the section that has the parts manuals in it. On the right side, there is a series of green bars. Click on the X for the Compact tractors-manual steering.
Use the serial number of your tractor to pinpoint the correct parts manual and then open it up by clicking on it. As long as you have Adobe Reader installed on your computer it will open for you. If you don't have A/R then go and get the free download by Googling it.
The drum may still be available from Ingersoll. If you need the dealer contact, then e-mail me. Used ones do show up on e-Bay from time to time. I'd contact Joe Hemmi of Joe's Outdoor Power in Michigan to see if he may have one sitting on a shelf. Joe parts out tractors all the time. This is a proprietary part so you aren't likely to find it any other kind of supplier.
If you want to watch for one on e-Bay, then use "Case lawn mower" as the key words in the search box. You will get the max number of hits with those words.