As Peggie said, finding the exact footprint would be tough, and it really shouldn't be the criteria for replacement. It's easy to drill new holes for the new head and fill the original ones. Who's going to be on their hands and knees to see if you did a perfect job filling those holes in the head anyway? When I replaced the toilet on my C36 I changed the alignment of it to better use the space while I was at it. My old adage: "if you have to replace, you might as well upgrade" worked in this scenario.Best of luck,Trevor