Powerband is only limited by emissions, and installation, but not engine design . Usually it is most noticeable in turbo diesels. The combustion speed, hence RPM, is limited so that you need more torque for power similar to gasoline model. Trouble is it's likely to be a derivative of the same gearbox, so that limits torque. The net result is that the max torque ends up closer to the max power, so smaller power band.
Since helis are constant RPM machines, i see no problem with a diesel installation. If anything a turbo diesel is ideal, since all can be matched for single RPM operation.
Mart