I answered him in the response (personally I'd probably use 5E because its the system I'm most comfortable with). Still, it got me thinking. While I love 5E, I've been wanting to do more OSR-related stuff too. While any of this could be taken and adapted to 5E easily, these are the sourcebooks I would use if I was going for an OSR fantasy East Asian/samurai-themed game
Ruins & Ronin - I talked about Ruins & Ronin a few days ago. I would use this as the foundation for my game. It's a modification of the Swords & Wizardry White Box, so its a good simple starting rules set. The Hero's Journey includes more classes that are adapted to the White Box. I wouldn't include them all but the Monk is included and the Thief would be a nice ninja class (if that's something you want in your game).
The following books provide some great setting material, though all are Basic D&D rather than OD&D (which means the DM's going to have to do a little work).
Qelong - The Lamentations of the Flame Princess adventure add some Apocalypse Now to your game. It's a hellish sandbox adventure that definitely has a Southeast Asian feel to it. For the foreign mercenaries in the setting I'd using some like Legend of the Five Rings Yodotai. As a side note Legends of the Five Rings is a good source of inspiration for an East Asian inspired game.
Mad Monks of Kwantoom - This pdf is massive. The book is actually several different things. It's an Oriental Adventures companion for Labyrinth Lord (Classic and Advanced). It's a Chinese-inspired monster manual. It's a tome of magic items. It's a campaign setting. Finally it's a rules set that allows you to generate dungeons and play DM-less D&D. This pdf is massive and has an amazing price of only 5 bucks (as of this blog post).
Yoon-Suin - Yoon-Suin is one of my favorite OSR books. It's a massive sandbox campaign setting. The setting is Himalayan (Indian, Tebetan, Nepalese), but it is those cultures as seen through the eyes of someone in Medieval Europe. The book makes no attempt to have any historical accuracy, but it really works. This book is everything you need for a game (minus core rules), but if you're wanting something more Chinese or Japanese in flavor, I'd just use it primarily to flesh out neighboring nations.
Essentially if I was going to run the game Dave mentions I would use Ruins & Ronin as the core rules, design a pseudo-Japanese nation and would make Yoon-Suin, Qelong, and the Yellow Islands' 1001 Pagodas of Doom the surrounding nations.
Does anyone else have any suggestions?