Complex sentence: Codenamed 4G54 and marketed as "Astron II", it was a development of the "Astron" engine (codenamed 4G52) fitted to Sigma, and it initially produced at 5000 rpm and at 3000 rpm.
Split into simpler sentences: Codenamed 4G54 and marketed as "Astron II", it was a development of the "Astron" engine (codenamed 4G52) fitted to Sigma. It initially produced at 5000 rpm and at 3000 rpm.