Yes, I contacted Mystery Ranch and they were not able to help me. I also contacted Marmot, and they do not have any frame sheets left.
Actually I am excited about rebuilding the frame sheet myself. It will be a fun project.
I bought another 20 year old Terraplane on EBay in almost new condition, only used once, for $152.50. Darned if it also didn't have a small crack in the frame sheet as well. I had heard that Dana had switched to a different material in later models.
I found a source for HDPE, and my hired seamstress thinks she can sew through it in order to install the stay sleeve.