Send it to these fine folks -
https://www.motorcycleradiators.com/. They will pressure test it, find the leak and repair it quickly. They also won't rip you off, and it will cost around the same as buying a used one off Fleabay that may be no better than the one you have now. They just did one on a Yamaha for me I thought was leaking, and they verified it was OK - the fault was a tiny split in a hose really close to the rad, so you couldn't tell.
Most auto radiator repair places won't touch ones off a bike, I have found.
