Call a mechanic to remove it and send to a radiator repair shop for pressure testing and repair.
Or there are stop leak products that can be added to the coolant that might work. One of the simplest is to add some ground black pepper to the coolant. The particles of pepper can plug tiny holes nearly permanently. I like to use a commercial product llike DIKE or BARS LEAKS
In any case, you must maintain the proper coolant specification in the radiator. Do not run the generator long with weak coolant or plain water or you will damage the engine from corrosion and cavitation. Of course the worst thing is to let the coolant level be low, so add clean soft water if you must, but replenish with the proper coolant mix as soon as you can.