PRIVATE entrepreneurs who were once scorned as ''scum of the earth'' in communist China have publicly lobbied for a greater say in national affairs at the just concluded Chinese People's Political Consultative Conference (CPPCC). Private businessmen and millionaires who were delegates to the national advisory organ made plain the days when a private economy was considered the synonym of capitalism were over. The delegates buttressed their claim by pointing out that the private economy was no longer a ''mere supplement'' to the state-owned sector in today's China. A report in the latest issue of the Economic Reporters quoted the delegates as saying that since China has set economic development as its national goal, it would be wrong to identify socialism with a planned economy and at the same time link the privatesector with capitalism. ''Since both market economy and planned economy are just means to achieve economic prosperity . . . it doesn't mean that the state-owned sector should therefore monopolise the economy because [some people] say that the state-owned sector is the main body[of our system],'' it said.