Good job. A new toilet is sometimes the answer, but not this time for you.
I recently replaced the toilet in my rental, in preparation for my new tenants, my parents. The old toilet didn't leak, but didn't flush well. It was about 12 years old. I think lots of letting the "yellow mellow" as some people do, and perhaps a less than stellar cleaning history, led to a lot of mineral build up in the siphon jetway (which is somewhat cleanable) and the trapway (hard to clean).
A new Glacier brand $88 toilet not only fixed the poor flushing but works 2x better than the older Kohler, and uses slightly less water. I also found that the wax seal was loose, and so it needed a new one anyway. Oh and the old seat was broken, the most recent non-family tenant didn't want to mention it. How you break a toilet seat is beyond me.