With only a 100 cc motor and with the room you have in your frame why not just a nice cylindrical tank that holds 2-3 gallons of water, put a pressure cap on it and run some ice in the tank, use a thermostat and use a small electric water pump to circulate the water from the tank through the engine. With a really good 100 cc motor you might be at 25-30 hp which make the water heat load max of about 10 hp so a 5 gpm pump (rated at 5 gpm at the pressure drop through your motor) would do it. The complexity of a radiator in a tank system just does not seem warranted for your application. If your must have some sort of radiator/heat exchanger then look into plate style heat exchangers, they are much smaller and more efficient than any of the "radiator in a box" systems and much smaller. There is a guy that runs a 350/400 Yamaha YZ-400 twin that is water cooled (this engine is probably good for 80-100 hp)and he uses a plate style heat exchanger and it is probably not bigger than 3 x 4 x 10 inches and it keeps his engine very cool, it is tucked up under his rear seat extension.
Rex