ASs long as it is a "JM" (real) Marlin, anyone with good mechanical skills and a degree of common sense, can make a good running cowboy rifle. Maurader(?) has a good tutorial on his site. If you want to spend mega bucks, then the following reply suggestions are very good. I have done my own as well as many others for pards, and all run as fast as the shooters can lever them, but NONE are short stroked or otherwise fiddled with.
Now, that said, the new "Remlin" Marlins, are mostly crap, and take LOTS of fiddling to get them to run even half A$$ well.
My experience, as well as MY opinions. Your milage may vary.
Al