I was in the living room when I heard a loud bang from the bathroom. There was
a crack going from a bolt hole in the tank right up the side. Water was
spewing out the bottom. It was lucky that I was home.

After lots of reading, I've seen the following:

-you can seal it
-you shouldn't seal it, it will always have the danger of a leaking disaster

-you can't use a modern tank as a replacement

-new toilets are apt to suffer from poor flushing and many models suffer from
leaks -- unless I get some $500 Japanese model

I wish I could just replace or repair the tank, I'm not concerned about looks
or anything. But it seems like replacing is the only option. The old toilet is
maybe 1960s, 1970s? The house is 1950s.

So my question is: are there pitfalls awaiting in trying to put a new toilet
where the old one had been? Will the bolts in the floor automatically match
up? Will the old flange need replacing? Anything else? Thanks.