You may be right that only Yamaha ATV engines smaller than 450 are made in China.
One guy posted that he thought his old Linhai 550 was a Yamaha designed engine.
But all the pictures I found of Yamaha 550s had a cover plate over the left side
of the engine. If you can find a picture without the cover plate, you can compare
it to the Linhai picture I will attach. Of course, comparing the service manuals
would be the best way to see if they are the same.
On a side note, Polaris actually ships the whole atv body/chassis from China
and then loads them with their own engine/trans. And now the entire Polaris ATV
is assembled in Mexico.
Here is a post about Yamaha Linhai, from back when smaller engines dominated
(2003) but Yamaha/Linhai may be making larger engines now:
Yamaha Motor has reached a 3-way agreement with the Chinese motorcycle engine maker Jiangsu Linhai Power Machinery Group Corporation (representative: Lu Haimin; Taizhou City, Jiangsu Province, China) and Yamaha Motor's motorcycle manufacturing joint venture Jiangsu Linhai Yamaha Motor Company Limited, known as "LYM" (representative: Lu Haimin; Jiangsu Province), to establish a new company to produce multi-purpose engines. The agreement will be officially signed in Shanghai on August 1, 2003.
The new company, to be named "Yamaha Motor Taizhou O.P.E. Company Limited" (O.P.E. stands for "outdoor power equipment") will be established in Taizhou City, Jiangsu Province in August of 2003 to produce four-stroke multi-purpose engines. The new company will be capitalized at US$3.3 million (approximately 400 million yen), 25% of which will be invested by Yamaha Motor Co., Ltd., 70% by LYM, and the remaining 5% by Jiangsu Linhai Power Machinery Group Corporation. LYM, itself a joint venture between Yamaha Motor Co., Ltd. and Jiangsu Linhai Power Machinery Group Corporation, manufactures and markets complete motorcycles and engines under the Yamaha brand name.
The new company will occupy facilities with a floor area of approximately 5,000 m2, and is expected to start operations in the autumn of 2004. It will produce six four-stroke multi-purpose engine models with a maximum rating of 12 hp. In its initial year, output will be about 30,000 units, and by five years (FY2008) total production is expected to reach about 400,000 units. The first-year sales for the new company are expected to be approximately 20 million yuan (approx. 300 million yen).
To meet these production plans, the new factory will be initially staffed by 50, and after five years, staffing is expected to reach 130.
At 2003, Yamaha multipurpose engines are being manufactured at our group company SOQI, Inc. (representative: Katsuhiko Sakuramoto; Kakegawa City, Shizuoka Prefecture, Japan) at a rate of about 200,000 units annually. With the establishment of the new company, this multipurpose engine production will be completely transferred to China over the next three years or so with the aim of achieving substantially reduced costs while retaining Japanese levels of quality. Yamaha Motor also hopes this move will step up sales volumes, by shipping the engines to countries all over the world, and strengthen the international competitiveness of its multi-purpose engine business.
This information has been copied from Yamaha Global homepage.