He's fought in Shooto, K-1 Hero's, Dream and the UFC. He tried out for the Japanese Olympic Wrestling team. His MMA record is 18-4-1, though it is worth noting that he has often fought at heavier weight classes than he should have.
Anyway, if you are to do only one piece of research about Kid Yamamoto, then let this video be it. It lasts for a whole 46 seconds, his opponent is Little Hercules (Kazuyuki Miyata) who is a good fighter, and...just watch the video.