My suggestion is to carry the doughnut up to your local auto parts place, sometimes the guys there can help.
Hopefully Casey can help, but otherwise you might just have to try a few different parts stores to find someone friendly. Parts store guys that deal with professionals (i.e., not the chain store sort) have gone out of their way to help me in the past.The auto parts store I brought it to said they wouldn't even try to match it up without knowing the oem application. Anyways the gasket is in multiple pieces. I actually put the gasket parts back in the car with lots of RTV and was able to keep it leak free to get through the last race but it has blown out again. I have checked 2-3 OEM applications that use a 2.5" donut gasket and they all are the wrong profile and too thick. I PMd Casey hopefully he gets back to me. I have sent at least 10 emails to forcedfed over the last month and called them multiple times and they just run me around in circles.