Quoted: Hi! I'd like to swap in a new recoil spring on my Mark II, but the assembly seems like it's peened together. Anyone have any tips on the procedure to do this? Any links to online4 instructions? Thanks!

You should have the booklet that comes with the gun which I think will verify the recoil spring comes as an assembly...the rod and "wings" included.
Sometimes those "wings" get bent or break off during a hasty reassembly...my 1952 "standard" model still has it's original spring...just curious why you need to replace it?

I used a lot of various Wolff springs, but never had to change a "captive" type such as the Ruger's.
Off the top of my head, I can't see how it could be done without some grinding and welding on the factory recoil rod, unless perhaps it can be unwound and swapped that way.
The price of the entire Ruger assembly is probably cheaper than just the Wolff spring itself, that's not to say Wolff springs aren't worth it, it's that Ruger's factory MKII springs seem to last nearly forever as is.

Ah, failures to feed...I need to back up a bit and check the usual suspect...
With a few hundred round of ammo fired, some more dirty than others, the magazines can hesitate enough to cause misfeeds.
It probably functioned fine when new, but has gotten worse as time went on?
I let one magazine "go" almost a few bricks to see how long it would last until it started having problems and found the sides of the follower was built up with a hard coating of crud, in effect making it oversized enough to slow it's upward travel, preventing the next round from being in place before the slide came back forward to chamber it.
It may even feel good by hand but this crud slows it down just enough to cause problems.
It requires disassembly to do it right, but you will have fun with the mag spring, guiding it gently back into it's hole while slipping the floorplate back on...wear eye protection.

Also, changing of the mainspring in a MKI, II or III is not for short tempered or impatient types.
Good luck, I hope you get her going again.