A vehicle engine system comprising: a gearbox assembly that includes a housing having plural interconnected gears (404, 1106) disposed between an engine (102, 302) side (804, 806, 808) of the housing and an alternator side (804, 806, 808) of the housing; a rotatable engine coupler connected with the engine (102, 302) side (804, 806, 808) of the housing, the engine coupler configured to engage a rotatable shaft of a replacement engine (102, 302); a rotatable alternator coupler connected with the alternator side (804, 806, 808) of the housing, the alternator coupler configured to engage a rotor of a first alternator, characterized in that: the housing is configured to be positioned between a previous engine (102, 302) or the replacement engine (102, 302) that replaces the previous engine (102, 302) of a vehicle and the first alternator and is configured to be coupled with the previous engine (102, 302) or the replacement engine (102, 302), the vehicle engine system also includes a flexible isolation coupling configured to interconnect the shaft of the replacement engine (102, 302) with the rotatable engine coupler, the flexible isolation coupling having an annular plate coupled with several elongated flexible bodies each connected at one end with the annular plate and connectable at an opposite end with a flywheel of the replacement engine (102, 302), and the engine coupler is configured to engage the replacement engine (102, 302) that is resiliently mounted in the vehicle and the alternator coupler is configured to engage the first alternator that is rigidly mounted in the vehicle to transfer rotation (412, 1008, 1010) of the shaft of the replacement engine (102, 302) to rotation (412, 1008, 1010) of the rotor of the first alternator. a gearbox assembly that includes a housing having plural interconnected gears (404, 1106) disposed between an engine (102, 302) side (804, 806, 808) of the housing and an alternator side (804, 806, 808) of the housing; a rotatable engine coupler connected with the engine (102, 302) side (804, 806, 808) of the housing, the engine coupler configured to engage a rotatable shaft of a replacement engine (102, 302); a rotatable alternator coupler connected with the alternator side (804, 806, 808) of the housing, the alternator coupler configured to engage a rotor of a first alternator, characterized in that: the housing is configured to be positioned between a previous engine (102, 302) or the replacement engine (102, 302) that replaces the previous engine (102, 302) of a vehicle and the first alternator and is configured to be coupled with the previous engine (102, 302) or the replacement engine (102, 302), the vehicle engine system also includes a flexible isolation coupling configured to interconnect the shaft of the replacement engine (102, 302) with the rotatable engine coupler, the flexible isolation coupling having an annular plate coupled with several elongated flexible bodies each connected at one end with the annular plate and connectable at an opposite end with a flywheel of the replacement engine (102, 302), and the engine coupler is configured to engage the replacement engine (102, 302) that is resiliently mounted in the vehicle and the alternator coupler is configured to engage the first alternator that is rigidly mounted in the vehicle to transfer rotation (412, 1008, 1010) of the shaft of the replacement engine (102, 302) to rotation (412, 1008, 1010) of the rotor of the first alternator. the housing is configured to be positioned between a previous engine (102, 302) or the replacement engine (102, 302) that replaces the previous engine (102, 302) of a vehicle and the first alternator and is configured to be coupled with the previous engine (102, 302) or the replacement engine (102, 302), the vehicle engine system also includes a flexible isolation coupling configured to interconnect the shaft of the replacement engine (102, 302) with the rotatable engine coupler, the flexible isolation coupling having an annular plate coupled with several elongated flexible bodies each connected at one end with the annular plate and connectable at an opposite end with a flywheel of the replacement engine (102, 302), and the engine coupler is configured to engage the replacement engine (102, 302) that is resiliently mounted in the vehicle and the alternator coupler is configured to engage the first alternator that is rigidly mounted in the vehicle to transfer rotation (412, 1008, 1010) of the shaft of the replacement engine (102, 302) to rotation (412, 1008, 1010) of the rotor of the first alternator.