By doing exactly what they are doing right now. Rocketdyne makes the engines that used to power the Space Shuttle, RS-25. RS-25 is quite comparable with the Raptor: very nearly the same thrust (about 500k pounds), higher Isp (450 in vacuum vs 380), but lower thrust-to-weight ratio (73 vs 200). The higher specific impulse makes up for the lower thrust-to-weight ratio (you need less propellant mass, but you have more engine mass). The most important aspect though is that the RS-25 is a reusable engine; for the Space Shuttle program, each RS-25 was used for an average of about 9 missions.
So, if Lockheed Martin decides to build a reusable rocket, then this engine just works off the shelf. No need for any new R&D.
