The new toilets do flush better, for two reasons.

First is the finish inside the S tube which is more smooth, so that DF does not stick, and second, and more importantly, the kind of valve that dumps water much faster.

You can take the tank off and change to new valves, but I feel a bad taste (not to your mouth) to fit a stinky old toilet with a pricy shiny new valve...

Ah, try this: pull the toilet, replace the wax ring(don't use the cheapest one). For $5 in most cases you get the problem solved.