Start in the mode POWER GENERATION SANDBOX:

Click the A icon in the bottom right a couple of times and the screen will turns black and the devil mode will be activated, enabling more RPMs.

Master enable: true
Water level: true

Insulation: 100%

In. water temperature: 100ºC
In. water contamination: 0%
Ambient temperature: 100ºC
Ambient pressure: 0.10 ATM

Blowby: 0%
Valve leakage: 0%
Flywheel mass: 0.50KG
Piston friction coef.: 0.00
Piston mass: 2.00G
Exhaust flow rate: 200.00%

Rotor mass: 0.10KG
Excitation: 0.00%

Clutch slip torque: 10.00 N.M

Now put the heat at 100%, throttle full, reverser full forward and put the engine to run using the starter to fwd.
In transmition go increasing the gears until the final ones S3 and 6and the generator speed will reache at 100k+ RPMs.

In the GENERATOR BOX PARAMETERS change the rotor mass to 1000.00KG, put reverser on neutral, turn off the heat as it will not be used anymore.

And finally set a lower gear to the generator runs the engine. The best gear I found is GEAR S3 and 3.

With this setup, you should be able to achieve about 15300 RPM.