Complex sentence: Most of the R66 design is based on the earlier piston engine R44 but a suitable turboshaft engine did not exist for the R66 so Rolls-Royce developed a new model of engine--the RR300.
Split into simpler sentences: Most of the R66 design is based on the earlier piston-engine R44. There was no turboshaft engine suitable for the R66, so Rolls-Royce developed a new one, the RR300.