As I live in Finland, its winter and I'm busy with studies till May it's good time to make plans what to do when weather allows (comfortably) working outside. I'll create a model of generator in Matlab Simulink and test different control systems. Probably simple PI-controller will be sufficient, though.

First thing to be decided would be the power transferring method. Axle vs belt. Both options have their own pros and cons.

Axle:
Pros:
+ ease of maintenance?

Cons:
- requires custom mounts to be done to electric motor pulley
- generator would become quite long but narrow

Belt:
+ electric motor already has pulley
+ could be used as 'clutch'
Cons:
- requires custom mounts to be done to diesel motor-end
- reliability? (Finnish weather conditions etc.)

The chosen method will determine the orientation of motors. The chassis of generator will be built on a trailer. When motors are mounted it's time to build control and automation electronics.