Can the 747 be re-engined as a twin?
Currently, the 777 has engines that have a max thrust of 115,000 lbf, for a total of 230,000 lbf of thrust. The 747-8 has engines with a max thrust of 66,500 lbf, each, for a total of 266000. And just to throw in for comparison, the A380, currently has engines each producing 72,000 lbf of thrust, and a total of 288,000 lbf.

Right now the 777 has some of the highest thrust producing engines in commercial service. This does not take into account the extra thrust that is required for engine out issues. With a 2 engine design you have to have enough extra thrust for loss of engine during take-off at max take off weight. Whereas with a 4 engine design, you only have to have enough thrust to account for loss of 1 engine. So for a 747 twin engine aircraft, as a rough estimate you'd need to have each engine produce at least 150,000 lbf of thrust if not more.