I guess if they want to keep the old toilet then you either need to find a replacement tank or fix the crack, huh!
I did some reading and it looks like at some point most toilets changed from 3.5gpf to 1.6 along with bolt spacing. If its that old of a toilet then it is definately going to be harder to find an old tank. I found a site though that sells them.
Toilet Tanks in Stock: Used, Salvaged Replacements
but... They are super expensive and look to be mostly sold out, but you could call.
So back to repairing the tank... I was thinking that maybe just using paint would work. Take the tank off and put a few thick coats of enamel on the inside of the tank. That would surely fill/cover the crack, then when you put it back in get some gasket material to sandwhich between the bolts holding it on or really big rubber washers.
Then again if the toilet is that old, I'd just upgrade to the lower flush toilet and be a little nicer to the environment and to save some money on the water bill. But that would require talking whomever is stuck on keeping the old toilet into getting a newer more environmentally friendly toilet :) Good luck.