You can replace the S600PC2.5 with a Toro Super700 PC 3.0, but the new head will be about an inch taller than the old one. The S700 is an exact-fit replacement, since the two heads use the exact same body (in the event that the old body won't unscrew easily)
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Otherwise, do some digging and install any (even taller) modern rotor with a 3 gpm nozzle.