I've never had to replace the friction disc, but if it's still there, try moving the spring clamp closer to put more spring force on it. But, yes the rod will have to come out to replace the disc. You can make a disc out of a piece of thick cork gasket material. If you take the linkage rod out, you will have to reset the max RPM. That is set with the U bolt clamp.
Also there is a compensating spring, #21. If it is missing the throttle will not stay where you want it.