Modular-type power plant for a rotorcraft, comprising at least one lift rotor, said power plant comprising: - at least one drive (3, 13, 23) chosen from the group comprising combustion-type drives and electric-type drives, - a main-power gearbox, comprising a transmission casing and a sprocket arranged in an inner volume delimited at least partially by said transmission casing, said sprocket having a degree of freedom in rotation about a main axis of rotation with respect to said transmission casing, said main axis of rotation being inclusive of or parallel to a rotary axis of rotation of said at least one lift rotor, said sprocket transmitting mechanically an engine torque generated by said at least one drive (3, 13, 23) to a rotationally fixed rotor mast of said at least one lift rotor, and - at least two mechanical connection interfaces (7, 17, 27), a first mechanical connection interface (7, 17, 27) linking said main-power gearbox mechanically to a first drive (3, 13, 23) of said at least one drive (3, 13, 23) and a second mechanical connection interface (7, 17, 27) being left free or linking said main-power gearbox mechanically to a second drive (3, 13, 23) of said at least one drive (3, 13, 23), - the power plant also comprising, for each mechanical connection interface (7, 17, 27), a pinion (8, 18, 28) engaging said sprocket directly, said pinions (8, 18, 28) being distributed in azimuth about said main axis of rotation, each pinion (8, 18, 28) having a degree of freedom in rotation about a secondary axis of rotation (Z2, Z2', Z2") with respect to said transmission casing, each secondary axis of rotation (Z2, Z2', Z2") being arranged in parallel with respect to said main axis of rotation the power plant being characterised in that each mechanical connection interface (7, 17, 27) comprises an interface casing (9, 19, 29) and reversible fixing means making it possible alternatively to fix and retract said interface casing (9, 19, 29) to and from said transmission casing. - at least one drive (3, 13, 23) chosen from the group comprising combustion-type drives and electric-type drives, - a main-power gearbox, comprising a transmission casing and a sprocket arranged in an inner volume delimited at least partially by said transmission casing, said sprocket having a degree of freedom in rotation about a main axis of rotation with respect to said transmission casing, said main axis of rotation being inclusive of or parallel to a rotary axis of rotation of said at least one lift rotor, said sprocket transmitting mechanically an engine torque generated by said at least one drive (3, 13, 23) to a rotationally fixed rotor mast of said at least one lift rotor, and - at least two mechanical connection interfaces (7, 17, 27), a first mechanical connection interface (7, 17, 27) linking said main-power gearbox mechanically to a first drive (3, 13, 23) of said at least one drive (3, 13, 23) and a second mechanical connection interface (7, 17, 27) being left free or linking said main-power gearbox mechanically to a second drive (3, 13, 23) of said at least one drive (3, 13, 23), - the power plant also comprising, for each mechanical connection interface (7, 17, 27), a pinion (8, 18, 28) engaging said sprocket directly, said pinions (8, 18, 28) being distributed in azimuth about said main axis of rotation, each pinion (8, 18, 28) having a degree of freedom in rotation about a secondary axis of rotation (Z2, Z2', Z2") with respect to said transmission casing, each secondary axis of rotation (Z2, Z2', Z2") being arranged in parallel with respect to said main axis of rotation