If the rubber boot itself is not ripped or broken, you might get away with replacing an oil seal. There's an oil seal ring (similar to a crankshaft oil seal) in a metal housing inside the rubber boot. You can gently pop it out and knock in a new one.
Its OEM number is: 40001720, and Corteco has it in its range on 12011443B
Replacing the rubber boot is pretty straightforward by the way.