I think you will find that it is a Case brand toilet. A photo or model number from under the tank lid or elsewhere in the tank will be helpful. Many Case lowboy parts are still available, and the answer may be as simple as you consulting the nearest Briggs dealer. However, the parts are often exceedingly-expensive. There are sometimes parts offered on the big auction site that people have discovered squirreled away somewhere without realizing their presently-outsized value, but we have to know for sure what we're dealing with to give more advice than that. Also, there is no reason to replace all the "innards" at the same time; let's start by figuring out what the issue is so you can decide what might need to be fixed or replaced, and how. (For example, no need to get a new fill valve when all you might need is some of the components of the valve.)
EDIT: Looks like Terry beat me to it, with a photo even!
