Since the crack is in the bottom and (presumeably) doesn't show, you should be able to use a quality repair product for a complete fix. I recently used a concrete
repair product on a birdbath that would probably work just fine on porcelain, but I will have to look up the name later. In the mean time, I'm sure your local construction supply dealer will have a suggestion.
If you do a repair, I would do it from the inside and make sure it's COMPLETELY clean and dry first. You might want to use sandpaper on the inside to remove any old
residue.
One more thing - If the crack is "connected" to one of the holes in the bottom, you must be careful not to lump up the repair material, or the tank parts might not be able to seal properly.

[This message has been edited by rpxlpx (edited September 05, 2000).]

I could not disagree with the above suggestion more. (Sorry, no disrespect). Patching old, cracked porcelin invites only a major disaster. It assumes that a crack is fully visable and repairable.

The big risk is a major system failure where the crack breaks wide open and water streams out into your newly remodeled bathroom and throughout the lower floors of the house. Imagine if this happens while you are at work, or on vacation. The ballcock will think the tank is empty and water will continue to follow into the bathroom.

A cheap insurance policy, buy a new toilet. For about $100 you get total piece of mind.

PlumberDrummer, that's a good point about a possible "flood" if the repair fails. You convinced me.
It it matters, the product I was trying to recall is Rezi-weld.

[This message has been edited by rpxlpx (edited October 09, 2000).]