How certain are you that you have a 1991 model? Do you have the model number of your engine? I would recommend replacing all the seals simultaneously, not just a single seal.
https://obparts.com/new-red-rhino-yamaha-4-6-cylinder-lower-unit-seal-kit-oem-6e5-w0001-51-00-6e5-w0001-f1-00/