Plumbing – Using a sealant to stop a toilet tank leak

That's a fairly popular fill valve. To open it:

1. Shutoff the water, you don't want a geyser.
2. Lift the float to the top position.
3. While holding up the float, push down on the top cap and twist the cap a quarter turn (I believe counter clockwise).

The cap will then lift off and you can inspect and clean any parts. Before reinstalling the cap, move any small parts to a safe location, get a small cup to place over the top of the valve, and turn on the water supply. This will jet water straight up from the top of the valve and clear out any debris.

Reinstalling the cap is the reverse of the above procedure. Put the parts back, position the cap back on top, lift the float, press down on the cap, and twist.

Note that if cleaning it doesn't work, it's fairly simple to replace this fill valve.

I was about to upvote the answer of @GreyBeardedGeek , because in general trying to find a match will be impossible. But then I read the word Toto. Toto is a different case: they have a corporate sustainability principle. The short answer is: call Toto at 1-888-295-8134. As for gaskets: one should come with the replacement tank, else is easy to get at any hardware store. Some PTFE plumbers tape is good also, if the supply line nut is metal.

And check the warranty also, it may apply depending on who you talk to at Toto:

TOTO warrants its residential faucets, bathroom accessories and cast iron tubs, excluding the acrylic side panel and accessories, (Product) to be free from defects in materials and workmanship during normal use when properly installed and serviced, for the lifetime of the Product from date of purchase.

The only real gotcha in this job is "don't drop anything". The tanks are made of ceramic and crack easily (not to mention do a bang up job on toes).

The only real gotcha in the warranty is that it is nominally non-transferable.