Can I replace a toilet tank, only? what do I need to know regarding tank size to match to existing bowl?
It is possible to buy just the tank which matches your toilet.  I have done just that when I smashed my tank trying to move the toilet to fix the seal.  The model number was printed inside the tank and a little internet searching found the tank, with all the hardware in it, could be shipped to the hardware store right beside my house for about $80, which was about 1/3 of the price of the new toilet.  Since the hardware was included, the repair was incredibly simple.

Try searching for any identifying information inside or outside the tank or bowl.  If you can find something, don't be afraid to spend 20 minutes looking, you could save a bit of money.