You're kidding right?
The toilets you have must use eight gallons a flush. Many of the low tank toilets use a large bowl refill to help with the flush.
Kohler's Pompton toilet that looked very similar was an advertised 9.5 gallons. It was a "selling" point.
They used lots of water very slowly. It was very quiet, but used twice what other toilets used in their day.
New toilets use 1.28 gallons. Even if that beast could be made to use 5.0 gallons, you are still using four times the water you could be.
If it was a Kohler Pompton, you would be using 742% more water.
Why would you continue to spend that much money every month on water? And repairs on toilets that use so much water when they are working right.