When you say you're looking for a "couple" mags, it sounds like you are thinking removable. While I'm sure you could modify the rifle to take these, you would need completely new bottom metal. Your best bet is to check Numrich for a follower and mag spring, or get that plate off and see if the original is still there. Those old military rifles all had fixed magazines.
An internet search indicates that the conversion of the 7.7 Japanese to .300 Savage was fairly common back in the day. Apparently, the mag was too short for 30-06, and these were the days before .308 Win came out. So, set the barrel back for the shorter round, ream the chamber for .300 Savage, and you have a hunting rifle. Of course, the Savage normally takes a 0.308 bullet vs. 0.311 for the Arisaka. Using the original barrel, it's either an undersize bullet or get a bigger expander for your 300 Sav die. I hear that the undersize 0.308 bullets often did OK. Good luck!