I changed out my motor long ago. I thought it was a semi-solid seal, not a gasket. Much like the seal on the heater box when I did the heater core.
I knew I had seen something like that before. Being an amateur radio operator, I bought some of this:
http://www.coaxseal.com/
Rolled up some beads to make seals after scraping off the old ones. Problem solved. Stays semi-soft in all temperatures for a long time.