As a custom maker I will chime in here.
The knife you want made is a project for an accomplished maker. Engraving in steel is not a beginners task. Such a knife would be a bargain at $500-600, and a known maker may charge $2000. The handle and blade are fine as you described them. The blade geometry ,grinds and tapers will be determined in the discussions between maker and buyer. Most accomplished makers have waiting lists, and special projects take even longer.
Be aware that if you re-grind a commercial knife you won't be doing any engraving on it.
Now, myself being a Clint Eastwood fan since he was Rowdy Yates, I can appreciate your ardor. I used to carry a .41 cal Navy Colt ( cap and ball) that I made, and wore a serape ( I have worn a Stetson for nearly 50 years, too). I clearly recognize the belt design of his holster, but admit that I never paid much attention to his knife. That is funny because I have seen the movie dozens of times.
FWIW, the gun in the movie is merely a prop. It was a .36 calibre Navy Colt 1851 reproduction that was bored out to take cartridges. Since no such gun actually existed, you have to guess at what they were trying to represent. I had a .45 cal 1873 Colt and matching rifle ( both took the same cartridge), which was what he carried in his other "Man with No Name" films. Since the GBU movie gun was a fantasy, I guess you can have/make whatever you think are the "right" weapons to match his.
My suggestion is that if you want the look, you get a commercial camp/bowie knife that looks similar in shape ( the handle won't be the same obviously) and make a sheath yourself to look any way you wish. This way you get a knife for $100 or less and make the sheath to you desire for maybe $30-50.
Now, if you want any further suggestions from me, I would ditch the cursive writing in the sheath. It would be difficult to do and probably won't look good. The best way would be to have "THE GOOD" carved in western letters ("the" over "good"). The snake could be carved into the sheath ( above THE GOOD) with much better detail than on the knife blade. The stitching would look funny IMHO, but that would be your choice ( certainly not my cup of tea). If you really wanted the design, maybe it could be stitched on the strap, and a small design stitched in the sheath between the snake and the words.
Edited to add:
The 1851 Colt pistol reproduction kits are fun to build and require no special skills. They are readily available for around $300. A standard pistol belt could be hand stitched to add the design you like.