Thank you all.
Indeed, some more details.
The kit in question is Revell Mercedes-Benz 300SL-24. In pictures attached you can see the hood alone and taped in place onto the chassis, in order to see the deformation.
Heating it in place, with a proper heat insulation of the chassis would proabably allow to obtain the correct shape: I'd need to be sure that aluminium foil is enough, as deformating the chassis too would be an unrepairable damage.
I could try to replicate the engine bay frame, but it's very complex. Some hot water would be needed anyway, to correct internal buldges.
Plastic response, in terms of deformation, is very non-linear with temperature. An air-blower is hard to manage, as the flow alone would bend the part.
I hope that those pictures may provide a better comprehension of the problem.
http://imgur.com/a/Gz9rh