some points/assumptions
The diesel is more efficient than the gas engine, even though they weight a little more, although not that much with the modern diesels. They can also run on biodiesel without modification, and with heated fuel pickups and heated fuel lines and a boost in temperature just prior to the injector pump (heat source can be electric or engine coolant), they can run on vegetable oil (non-hydrgenated canola or soy being best).
(I keep forgetting that the hybrid electric motors use a wide and high range of voltages.)
Using a generator with rare earth magnets, one could have the engine drive the shaft with the magnets on it, to have a brushless generator - lots of juice for the weight and less upkeep.
A generator is heavier than an alternator (magnets vs. windings) but the d.c. doesn't have to be converted from a.c. to d.c..
Depending on the load requirements, one might be able to take a page from the off-the-grid hydro projects and modify large alternators from self exciting to rare earth magnet. On the other side a self exciting alternator can be modified to have a simple control circuit for the exciting load, such that one can vary the alternator load to match the gas motor to keep it operating at its most efficient.
I guess I need to find out more about these new electric motors, as in weight, voltage, current, torque, rpm, etc..
I am much intrigued.