You can emulate the look of Lucas' carbine "on a budget" with a used Rossi 92. They've been available stock--or can be retrofitted with the Rossi part that's out there--with a close approximation of the rounded "D" style which was one of two of the loop-lever styles he used. Early style Rossi large loop:
He also used a more squared-off D which looked less refined/more backyard gunsmith, but must've suited him better operationally (as it was later episodes IIRC)...and would need to be custom-made now. The "John Wayne" style mentioned is more of an oval--also available at times from Rossi and the Italians (Chiappa made Taylor's and Cimarrons as the "Eldorado" for instance, and on some Win 94s over the past few decades) and will not give you as much of the larger-loop Rifleman look--like this current Wayne style Rossi:
Legacy Sport's (LSI) Chiappa-made "Puma" also had a fancy (autograph, emblem etc) Rifleman commemorative that was a reasonably precise Lucas McCain facsimile (with the more rounded/earlier style lever, not the squared-off).
Functionally, the large loop is a mixed-bag proposition, actually increasing
cycle time in normal use. The plus side - greater ease of use in cold weather with gloves . I swapped a large loop from a 16" Rossi 92 trapper (which is how.a lot of the looped Rossi 92s came) to a regular 20" carbine - 1) I wanted at least one of my Rossis with the Rifleman look/cache, and the large loop seemed to suit the longer 20" gun better than it did the 16",.and 2) winter shooting.
As for the TV show's rapid fire modification (operable set screw): yes it can be--and has been--done, as mentioned. I wouldn't--just an accident waiting to happen IMHO, unless you are a very well trained--and tall--"showman" by trade...or unless you want the detail of the look of the set screw but fixed/backed-off
so it can't be "actually actuated." Many episodes showed Lucas adjusting/backing the screw off for "normal" shooting (precision/slow target practice, hunting, etc). In one particular episode, a kid was shown accidentally shooting and killing a buddy by actuating the lever when the gun was already cocked--and of course chambered--with the screw tripping the trigger, as designed when "screwed in" but with tragic consequences. Noteworthy "gun safety" episode.