Hi wildmetal, are you sure you actually have a problem? If the overflow tank is at or over the recommended level and you run the bike hard it will overflow a small amount.
I would ensure the radiator is full, the thermostat is in and working and the overflow bottle is at the correct level. Take the bike out and run it hard then leave it overnight before checking the level in the overflow bottle.
Make sure you also check for leaks in the hoses, they may be getting worn and could be leaking at the entry to the engine or radiator. Mine used to occasionally overflow after a thrashing but it was never an issue.
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