The plan is to eventually use a modern replacement unit, but in the meantime I wanted to give this 10" Master Plumber 211L toilet tank repair kit a try, and it was a perfect fit/replacement. Check out my video clip.
I did not use the supplied flapper as the old one is still working well. Will keep the new flapper as a spare.
Moving forward, I guess the modern replacements will work out cheaper when purchased as a set of two. Nevertheless, I'll still give a 5 star rating because this does the job well.