Complex sentence: It was a jet-powered development of the piston-engined Saab 21 which, along with the Russian Yakovlev Yak-15, was one of only two jet fighters to have been successfully converted from a piston-powered aircraft.
Split into simpler sentences: It was a jet-powered development of the earlier piston-engined 21. Along with the Russian Yakovlev Yak-15, the 21R was one of only two jet fighters to have been successfully converted from a piston-powered aircraft.