THE Macau Government has agreed to underwrite a $2 billion loan for the airport at Taipa. Governor General Rocha Vieira said the guarantee had been approved by the Consultative Council and was expected to be repaid before 1999, when Macau reverts to Chinese sovereignty. Speaking before leaving for an official visit to Portugal, he said a further loan of $1.2 billion was the subject of ''mutual understanding between the Chinese and Portuguese Governments'', but refused to elaborate.