HI Sean,
I would think that the cork washer on the breather sleeve has shrunk/ dried up while the bike was off the road
Pull off the timing cover and see if theres any end float on the sleeve which sits onto the end of the camshaft gear
A new washer needs to be compressed when fitting, several thicknesses come with the gasket sets,
fit the next thicker washer than the one that allows the inner timing case to fit against the crankcase with no end float
HTH
John