Somehow I doubt you will even be able to find a tank to replace the broken one but power to ya if you can.
I'd place the odds at better than even you will be forced to buy a new toilet.
If you have to do that a Toto toilet would be a great choice.
The problem is that I don't think it will do anything to either make your problem better or, worse.
Your problem as you state it is a line problem where the plumber feels you have an older line that is in poor condition.
What some times happens in these older lines is with lower flows of liquids it leaks out of the pipe before reaching the end of the pipe.
This causes solids to accumulate in the pipe and drying out resulting in a packed section of the pipe which resembles a hot dog or, sausage blocking the entire pipe. We drain cleaners refer to this as a dry pack. This is often aggravated by low flow toilets but really is a line problem.
Personally if you have not had the line camera inspected to see what is wrong with it I would do so and see about getting the line repaired.