Simon Yuen passed away on December 17th, 1980 and here is the evidence via his obituary from a well-known Hong Kong newspaper. The first article disputes what was printed about him in the English language, and his erroneous date of death. The second article is his obituary. Translation provided by ???.
(This serves as an excellent example for editors here on the use of secondary sources - all of which in English get this date completely wrong. Always assume everything is wrong until you see it in the movie credits, or on the poster, for yourself. Or in this case, in the obituary...)

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Translation:
The Kung Sheung Daily News (????) Sept. 11, 1979 -
Mr. Yuen Wo Ping yesterday announced he was getting married on Oct 31. Yuen Xiao Tian was very happy about his son’s marriage. Because his right hand is not feeling well, and he has stopped acting. His role in “Lin sha yong” (a.k.a. "The Magnificent Butcher") was substituted by Fan Mei-Sheng. “miracle” shown in the movie on the right would be his last movie playing a major role.
He also said his oldest son, Yuen Wo Ping, is 36. His marriage was delayed because he has been helping taking care of siblings. That is very rare. His future daughter in law, age 26, will be a beautiful and good wife. She came from Taiwan. After the marriage ceremony in H.K., they will have to register their marriage in Taiwan as well. They will live in H.K. Yuen Wo Ping is busy directing "Lin sha yong". His dad, Yuan Xiao Tian, is in charge of arranging his marriage reception and misc. family business.

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Translation:
The Kung Sheung Daily News (????) Dec 18, 1980 -
Well-known wushu director Mr. Yuan Xiao Tian passed away yesterday at 9:35am at Kowloon hospital, at the age of 69. He was survived by 11 kids, 4 daughters and 7 sons.
He has been working in movie business for over 40 years and directing wushu in movies.
Since his first movie “ wong fei hung”, he has been involved in acting in 90% of his 80+ movies. He is also the first movie wushu director and founder of action movies.
He has many students, including the famous Tang Jia and several of his sons. After his performance in “drunken fist” and “snake hand” directed by his elder son, Yuan Wo Ping, he became very popular. He even set the record of acting 8 movies in a year and was the top guy at the time.
Earlier this year, because of his overwork, he was admitted to the hospital and had a surgery to remove his lung tumor. After returning home, his condition got worse and he was hospitalized one month ago and passed away yesterday.
His funeral was scheduled on 19th at noon.
(Note that Simon Yuen's age at the time of his death is listed as 69. This is in Chinese age reckoning, whereby one is born at age 1, and age increases by one on each Lunar New Year. Even though Simon Yuen was 68 at the time of his passing, by European reckoning, had he passed away a few months later, after Lunar New Year, he would have been 70 by Chinese age reckoning.)