AS the Chinese Government celebrated Deng Xiaoping's 90th birthday yesterday, Hong Kong observers said it was well prepared for the end of his era. The chief editor of Contemporary monthly, Ching Cheong, believed the authorities had almost finished preparations for Mr Deng's death by restructuring the People's Liberation Army (PLA). ''[President] Jiang Zemin has taken over the control of the Army for more than one year and he has just announced the promotion of 19 officers to full generals,'' he said. Mr Ching said that both psychologically and in practice, the authorities were prepared for Mr Deng's death. Hong Kong affairs adviser Xu Simin also said Mr Deng's death would not cause any earthquakes, as the PLA was under the control of Mr Deng's old subordinates such as generals Liu Huaqing and Zhang Zhen. He said the political scene would also not be affected as Mr Deng had resigned from all his official posts. But the chief editor of the Nineties monthly, Lee Yee, believed Mr Deng's death would trigger a power struggle within the Government as his successors such as Mr Jiang did not have a strong foundation. He said both remnant Maoists and liberals who wanted to overturn the official verdict on the June 4 massacre might pose a challenge to Mr Deng's successors. The three commenters agreed that it was hard to tell the health and the political fortunes of the patriarch. from the birthday celebrations.