CHINA's call for international aid to help repair its most famous historical monument, the Great Wall, has come too late for many sections of the original structure. ''Some parts have been ruined beyond repair, occupied by local industrial and agricultural units, or have totally disappeared,'' said deputy secretary-general of the quasi-governmental China Great Wall Society, Ma Keqiang, in announcing the new drive for foreign funds. The worst affected areas are in the central provinces of Gansu and Shaanxi as well as Inner Mongolia, where huge sections of the Qin dynasty mud and compacted earth wall have either been eroded by wind and rain or destroyed by those living nearby. Many sections of the more stable stone wall built in the Ming dynasty have also been lost forever, demolished by farmers looking for construction materials for their houses, pig pens and other buildings. It is difficult to say how much of the 6,000-kilometre structure can be repaired. Much of the wall is located in remote, sparsely populated areas of central and northern China and will require considerable manpower, time and resources to renovate and, more importantly, maintain successfully. The bulk of the 100 million yuan (HK$90 million) China hopes to acquire from its international fund-raising drive will probably be devoted to the best known sections around Beijing and at either end of the wall; around Shanhaiguan on the coast of Bohai and Jiayuguan on the arid Gansu Plain. Many of the wall's most famous sites north of Beijing, such as Badaling, Mutianyu and Jinshanling have already been restored during the 10-year ''love our country, repair our Great Wall'' campaign initiated by paramount leader Deng Xiaoping in 1984. But more funds are needed if the link sections are to be repaired and the famous sites maintained in good condition. Officially, the main reason for repairing the Great Wall is to restore a sense of national pride in what is probably the largest man-made structure in the world. However, there are equally important material considerations, namely attracting more overseas tourists and the dollars they bring to China's hotel, restaurant and travel trade. The Great Wall is probably the biggest single tourist draw in the country and the authorities have an obvious vested interest in making more of the wall accessible to the hordes of eager sightseers. Millions of yuan has been devoted to building tourist facilities at the wall, such as the cable car at Mutianyu, and in constructing new roads to make the new sites more accessible. The new road to Badaling, for example, includes a centre lane specifically designed to whisk visiting dignitaries to the wall in 40 minutes from downtown Beijing - about half the time it would normally take. The construction of tourist facilities at and around the Great Wall has been a success in that the number of visitors has increased substantially each year since the low period of 1989. But that success has brought more problems. The influx of tourists threatens to seriously damage those sections of the wall open to the public, and unless the authorities can devise a scheme to control or defuse the flow of visitors, many of the best-known sites could suffer permanent damage. One section of the wall at Simatai, 150 kilometres northeast of the capital, has already been destroyed because of the desire of over-zealous officials to attract more tourists. Not long after more than 60,000 spectators crammed on the wall and the surrounding hillsides to catch a glimpse of British motorcycle stunt rider Eddie Kidd's leap at Simatai last year, a 30-metre section of the wall collapsed, much to the embarrassment of the authorities who organised the stunt. No similar spectacles have been permitted since. It may turn out that the more money the authorities spend on turning the Great Wall into a tourist attraction, the more they will have to spend on protecting and renovating it.