A couple of ideas pop into my mind, but I have to tell you that they are not as good as replacing the tank and bowl:

1. Install a new fill valve with adjustable water level setting.

2. Insert a gallon of water container into the tank, space permitting. I don't like this method, it might interfere with the daily operation of the flapper, and I don't know the size of the tank.

Like I said, toilets are so inexpensive and there is no reason to keep the old one. Replace it.