Blecch. $25 in parts to fix a hissing ballcock? Sucks.
First, let's make sure that it's not hissing because your tank water is draining around, say, a leaky flapper and it's hissing because it's refilling the water that's leaking out of the tank and into the bowl. Easiest way: mark the water level in the tank with a pencil. Turn off the water to the toilet for a few hours. Check the tank and see if the level is exactly the same. If it isn't, your fill valve is hissing because you're losing water and it's refilling the tank. Most likely, this indicates a need for a new flapper. You can buy the Kohler flapper or you can buy the Korky replacement, which is probably higher-quality. The Korky part for your toilet is No. 58BP (the "Flush Ball Flapper"), assuming that your toilet is the K4520 version of the Wellworth, and assuming that someone hasn't changed the flush valve. Lowe's has it here: Lowe's 58PK Page. Don't worry about the letters after the number; it's a common thing with Lowe's to change the BP to PK on Korky product numbers. If this looks like what you have, it's the only flapper that looks like that. HD doesn't carry it. Grainger does, but only for mail order. Korky instructions for it here: Korky 58BP instructions
But if it is indeed some problem with the fill valve, then let me make a suggestion. It appears that the later models of this toilet use a Fluidmaster 400A fill valve. This means that your toilet will take basically any plain-vanilla aftermarket fill valve. Lots of the plumbers on here swear by the 400A, but they are more of a pain to install than the Korky 528. If it were I, and my girlfriend, say, asked me what to do, I would tell her to go to Lowe's (or HD or ACE) and plunk down $9 for the plain-vanilla Korky 528 with the white cap. (I have five toilets with Korky 528s in our house.) Then I would tell her to replace the ballcock with that valve. It's pretty darn easy. It's easy to adjust the water level, then you just lock it in place and forget it. There's a video here about how to do it: Korky 528 replacement video And here are the written instructions: Korky written instructions. The thing is easy to install, has a five-year warranty, and the manufacturer has really nice people in Wisconsin who will help you do it over the phone if you need help at 1-800-LAVELLE. (Lavelle makes Korky products.) There's just no reason to mess with a ballcock if you can replace it with a valve like this. And if you ever have a problem with this valve, all you have to do to fix it is usually to open it, remove a little cap, rinse it off (or, years from now replace it with a $3 part you can get at Lowe's), reassemble and you're back in business. Charmingly-low-budget Korky 528 Marketing Video If you do go this route, note two these two tips: First, because of the anti-siphon action of this valve (to prevent tank water from siphoning back into your house water supply), the valve drips water from the top a little, as shown in the instructions; ignore it. I mention this because it ends up being a common question: why is my 528 "leaking"? It isn't. Second, be sure you put the refill tube on a clip on top of the overflow riser, as instructed. (Terry always says to "daylight" the refill hose above the overflow riser.) If you just shove the tube down the overflow riser, it will siphon tank water out of the tank and down the overflow riser into the bowl, leading you to ask why your tank water level is dropping.
If you really like more of a challenge, I know that many plumbers like that 400A (Made in Mexico while Korky is made in the USA), but it's significantly more steps, and significantly more complicated, to install. No big deal for plumbers, but then soldering copper pipe is no big deal for plumbers.
Anyway, in sum: I would make sure it's actually a fill valve (ballcock) issue first, and, if so, ditch the ballcock and install the Korky 528. [Expect some of my compatriots to disagree about which aftermarket fill valve to use, the 528 or the 400A, because each has its proponents, but that's my 2 cents and I'm confident they will agree with the rest.]