Katayama Sangyo Co., Ltd., Nagoya City, 1951-63
Katayama Sangyo was formed immediately after WWII in 1947 and initially produced agricultural engines before switching to motorcycle engines in 1950. According to the Japanese language Wikipedia, they produced engines both for themselves as well as other motorcycle manufacturers (ITO Engineering is offered as one example), and they sourced the frames, running gear etc. from other local suppliers. Thier motorcycles were sold under the Olympus (オリンパス号) brand beginning in 1951, and offered in both 4 stroke and 2 stroke versions. While Katayama Sangyo survived the Ise Bay Typhoon of 1959, they faded from the scene fairly quickly and the company was liquidated in 1963.
Worthy of note (to me at least) is that in 1962 they may have built, although never put into production, the first 3 cylinder air-cooled motorcycle 2 stroke engine (the Scott 3 cylinder was liquid cooled) thus predating the big Kawasaki and Suzuki triples by a wide margin.