If your local hardware store has the korky 528T fill valve, it is the same as the original valve. If not, get the Korky 528mpk at Lowe's; it is identical to the new Toto universal replacement fill valve. You might try draining the water and removing and reinstalling the old valve. Check the gasket on the inside of the toilet, rinse off, etc. Maybe lightly sand the inside of the tank around the hole to make sure it is smooth for a good seal. But the gasket may just be done, partiucularly if exposed to chlorine or bleach on the tank water. Hope this helps.
PS you need the 528T or 528 mp. The regular 528 white cap one will not work. You need the blue or silver cap one.