This is kind of a list, and a story of caution.
Brownells sells a quick take down knob that is great. They sold a plastic widget to keep crud out of the action during cleaning. Get a crown protector made of brass. If you want to go all the way to stop the bounce look at Wild West's Happy Trigger. Go to Leverguns.com fixes on the "Marlin Jam". Resist temptation!
A solid firing pin swop and loosing the crossbolt safety modification may become a liability down the road. I have a web sight for a very cool SS replacement mag follower that seems to need some finding at the moment. Alloy ones will gall and wear unevenly in the tube[everything here requires maintenance for sand/grit]. The stock ones will shred.
If you can afford it get the AO rail. I have the first generation and love the peep. Other wise Brownells will sell you a full buckhorn that will slid right into where that POS folder resides. If you do not like the AO or the buckhorn a Marble tang can be had for about the same price. Take the disk out and you have a ghost ring. Invest in a Merit #42 [with the right ammo an you have a real blaster.
Brownells did sell a stock up grade to walnut for the "C" guns. It means cutting a slot for a tenon, a cap, some screws and eighty $ for the wood. The barrel band gets lost so there are some holes in the mag tube and barrel. Yup you need to have the barrel cut for a mag tenon and buy a screw.
You are correct. I fired some with some Winchester Winclean's 125, and some Winchester 125,,158 JHP's after cleaning and then some Winchester 145 Silvertips all at 25yards and a clean gun between boxes. Soooooooooooo. I got out the 140FTX Hornady levereolution. I set the mark for 50 yards fired a sting of five form a cold barrel and let the gun cool down. I moved the target to 75 yards and and zeroed the rifle. There is no doubt this rifle will go 125 yards in the right conditions if I do my part with iron sight with this ammo. I have all the data I need to change back to "regular' ammo if need be.
Note to self. Do not work up a cool shooting light fast handling lever gun. Then go to the range and shoot it. Bring it home and brag about it, spread the targets out on the kitchen table and explain it all " to you know who." over a glass of lemon aid. Do you see where this is going? Yup!