Andre':

Your biggest problem will be cooling. This argues in favor of a helicopter engine set, and two stroke as they generally are the coolest running.

The engine itself, unless you use an engine intended for ducted fan, isn't going to run the 25K to 30K you'll need for efficient power generation. Pick the engine you want, check it's torque peak rpm, and gear it to run the generator around 30K rpm.

Then if you use a "Brushless" motor feeding into a full three phase rectifier you'll have a pretty good generator set. You can select the motor (generator) to get the voltage and current you want. Bearing in mind the engine power being available to turn the generator at that output.

I suggest using a brushless motor, it will give you the high current through a lot longer life than a brushed motor would, as well as having a higher current capability.

Bill.