Generator using a 2-stroke nitro engine
What is the power rating of the engine? Roughly speaking, you would choose a '12V' rated motor of similar or larger power rating. So if the engine makes about 2HP as stated in its datasheet, you'd choose a motor of at least 1500W, about 125A@12V which seems kind of insane to me. With those kind of currents, using a 24 or 48V motor might be more realistic.

The example you show direct-couples the generator (motor), but you have the option of choosing a lower-speed higher-torque motor with a belt or gear reduction too. If you do use reduction you have more flexibility in choosing an optimal shaft speed for your motor, and choosing bigger motors with higher outputs.

Either way, take the raw generator output and post-regulate it as needed using one or more DCDC converters.

More: a hybrid approach might allow a smaller gas power unit. Example / inspiration: https://www.vice.com/en_us/article/wnxjaq/a-canadian-startup-wants-to-replace-drone-batteries-with-a-gas-engine-pegasus