Looks like my ancient Mansfield toilet needs to have the flush valve
seal, possibly the entire assembly replaced. Although messy doesn't
appear to be too difficult a job.

Curious as to whether that flush valve assembly could be replaced with
something like a Fluidmaster? Would it be a straightforward substitution
or are there pitfalls likely to be encountered. Are these things at all
universal?

At this point in time I have not yet removed the valve so I don't know
exactly which model it is. All I can tell now is it is mostly plastic.