Koku is a set of basic Samurai warfare rules with a mechanism for purchasing your troops.
The prolific Richard Bradley has a set of free wargames rules for samurai skirmishes.
No Dachi is a set of free wargames rules for Samurai skirmishes.
The older I get, the more I like simple rules sets. Here’s one for Samurai Skirmishes.
From the introduction:
Sensō is a skirmish game, 1:1 figure scale, for 25mm miniatures. One scale inch represents six feet and one turn roughly approximates one-two minutes. Bases should be square for foot, approximately one inch. Mounted can be 1” x 2”. The game is WYSIWYG (What you see is what you get); figures are armed and armoured as they appear.
The Water Margin is a set of free wargames rules for oriental skirmishes.