there's nothing special about 240K L24 pistons. Absolutely no sense going out and sourcing them seperately. They are the same as L16 pistons. R30 Skyline L24E engine had flat tops and fly cuts from factory but they haven't been available new for many many years.
Using the L18 85mm pistons from the engine you got the crank out of is a much better idea. With longer L16 rods you can mill the l18 pistons to flat tops anyway. You gain capacity and compression this way, and improve the rod:stroke ratio. It's a win win win situation for next to no cost. And it's perfectly safe and reliable.