If it is the center push type, I think you can just replace the entire plastic flush assembly, if you can find one that fits. Seen a lot of them at big hardware shops. But they are all 'standard' size, if yours is a designer non-standard type, may have to go back to agent to buy.

Just unscrew the whole thing out of the tank, and run around see if someone can sell you one.

Before that, you may want to try this -
Look into the water cistern
right at the bottom of the center flush assembly, where it connects to the bowl, you will see a piece of black rubber washer.
Remove the entire flush assembly, it should be just a twist n pull action needed.
Then clean the piece of rubber washer and fit back the flush assembly.
Usually there is dirt gathered below the washer, causing water to leak into the bowl.