The original G.91 versions were - they had a single Bristol Orpheus turbojet. The G-91Y variant, which was based on the G.91T trainer fuselage, had this replaced by 2 afterburning GE J85 turbojets which increased thrust by 60% over the single-engined variant. Further structural modifications to reduce airframe weight increased performance further and an additional fuel tank occupying the space of the G.91T's rear seat provided extra range. Combat manoeuvrability was also improved with the addition of automatic leading edge slats. The image below gives you a bit of a side-by-side comparison: