as others have said, if its running good for you, leave it bee!
i had a stock zen 26 that i had for about 10 year, used it regulary, only thing i did was a piston ring change after about -5 years & a head gasket ...other than that, a new spark plug here & there, never any issues...just make sure head bolts are checked regulary as with it being 2-bolt...have a tendency to come loose if not properly tightened