I'm new to this forum & have been searching the Internet to try & locate a replacement for my 1976 fresh green toilet. I think the toilet bowl is in pretty good condition but my problem was a leaking tank. I know very little about plumbing- so I had a plumber come to take a look. He took the tank & toliet apart & found a hairline crack in the bottom of the tank. It's on the outside not inside of the tank & doesn't appear to go all the way through. I asked if it could be repaired & he said no. The crack isn't huge- with all the chemical compounds these days I just don't understand why epoxy wouldn't work. The inside tank components were working but need replaced & no one around here seems interested in helping me. The answer I've received when I've asked is "just buy a new white toilet." Well the sink & tub are also fresh green & I like for things to match. There are no chips or cracks in the lid or bowl. I see where you said you have a fresh green Kohler- do you still have it? Or do you have any experience patching hairline cracks in the outside bottom of the tank?
As long as the crack doesn't extend to the area of the bolt holes, I don't see why you can't repair it. The fact that it's leaking indicates that the crack goes all the way through, even though not necessarily visible on the inside.
Here is what I would do. Buy the new tank components. There's no reason to use OEM parts, unless you display this as part of a non-functioning collection. The new fill valves are better than the Fresh Green generation float ball type. The basic universal Fluidmaster or Korky will work fine. Buy a Fluidmaster or Korky universal adjustable 2-inch flush valve with the red flapper. Finally, buy the Kohler tank-to-bowl gasket, Item #K-GP51487, pictured below. With the tank components removed, thoroughly scrub the outside bottom and entire interior of the tank with baking soda. Rinse thoroughly and allow to dry, preferably outside, weather permitting, for at least 8 hours. Paint the inside of the tank with a high quality waterproof marine/swimming pool paint, following the label directions. I would use at least 3 coats. On the underside of the tank, patch the crack with silicone caulk. It doesn't matter whether you use white or clear since it's on the bottom and won't show. Draw a thin bead along the crack and then work it well into the crack with your finger or a small putty knife, smoothing down and removing any excess. Let the silicone dry for 24 hours, reassemble and you should be back in business. WARNING: If the existing crack is close to the bolt holes, it will almost certainly spread when you tighten the tank bolts. BE VERY CAREFUL.
I noticed you said you know little about plumbing. This project is very basic and the instructions included with the new tank parts are very clear.
I still have my Fresh Green elongated Wellworth. It was still new in the box when I bought it. I installed it just long enough to check it for damage and functionality. It has not been "christened." The tank components had deteriorated from years of being in storage, so I replaced them. I also bought a new plastic Bemis Color Direct seat for it. I'll sell it for $500, shipping included.