Marlin magazines can be fitted for use in JW-14. Reports vary on ease, but my own experience was pretty straightforward. I fitted a hi-cap promag to work in a JW-14. Steps involved fitting the magazine to magazine well:
i) I had to remove some material all 'round magazine top to get it to fit into magazine well.
ii) Modify where the magazine hasp catches magazine. On my example the promag had a full width catch but JW-14 has a narrower hasp so I had to change that to look like the JW-14 catch (the edges of the catch were sanded smooth to magazine body leaving only a centrally located catch **pictures are worth more than 1000 of my words so I'll try to snap some**)
On the rifle itself I had to inlet the stock around the magazine well so the promag could fit flush to reciever. Less than 1/4" of wood removed all around the magazine well.
I haven't any steel Marlin Mags floating around to check what would be required. I also have suspicions that Squires Bingham magazines might fit, but am fresh out of those too.
You can also purchase replacement magazines specifically for the JW-14. I purchased one from SFRC, but they show out of stock on their site currently. There may be other dealers with them.