Welcome to the forum and congratulations on receiving your grandfathers Model 80.You are a lucky man. To be honest your chances of finding a rear sight for your Model 80 are remote. I am told that from time to time they appear on auction sites but are never cheap. Because they can be removed easily they also get lost easily. There must be a lot of Model 80s out there without sights. Don’t give up. Keep looking. In the mean time I would consider putting a telescopic sight on your rifle. Take your rifle to a good gunsmith and ask them to find a mount that will fit the non standard size of the sight rails on your rifle. Various models have a degree of adjustment that will allow a firm fit. Once you have the mount (or mounts) you can select a scope.
We would love some pictures of your Falke including one of the serial number on the loading tap.