Unfortunately if the release has gone (and its one of their common downfalls) its on the cam gear - ie you have to tear the motor down to put in a new camshaft/gear assembly.
Hopefully its just the valve clearances too loose.
for starters, could you please post a couple photos of the motor and the model type code number. it could be on top of one of the valve cover (if its OHV which I hope it is) or somewhere on the metal cowl (if its one of the old opposed boxer motors)
