It sounds like there may be an obstruction in the line that may be restricting the flow of water out of the toilet. I would check that out first.
Many people who buy new low flow 1.6 gallons per flush,(GPF)toilets, (that are required by Federal law), are not happy with them, claiming there is not enough water to flush the toilet without multiple flushes.
If I had to change an old toilet, I would look for another old toilet. They can be found.
As a Building Inspector, I have seen new homes with the rough ins for a old toilets, (further away from the wall), new toilets installed for the C.O., and then afterward switching it to old toilets.