there are a bunch of different ways to do a parallel hybrid system but honestly the starter motor on the 240z is super old ive seen motors that size these days be able to put 20-30hp + out. If using capacitors for quick burts then your power pack can be fairly light and you can get a good advantage on power to weight also while increasing overall efficiency
would need:
the motor
some inverter / ac/dc dc/dc or whatever converters are needed based off your power source, regen, and your motor
energy storage
batery/cap management
potentially some sensors and other stuff to make sure everything plays nice
if its capacitors might not even need a charging circuit.
think the hardest part right now would be making sure it all plays nice and maybe making a clutch for it, these days you can find motors that can operate in so many different conditions, if the right one is found and an adapter and potentially a clutch and a grear box might need to be made....
vonnen does a cool kit for porsches where they pretty much integrate everything into the bell housing. this would be a way cooler peice of kit, but could cost quite a bit more and be more complicated to develope.