Search the forum there is a how to available. Since you have a first gen 126, it has a OM617, which is the same engine as in a late 70s/early 80s 300D. The OM617 injection pump is similar to the pump on an OM616 found on the late 70s/early 80s 240D (except the OM616 is a 4 cylinder). That is why the stop actuator will fit.
Be very careful installing the valve on the IP, as it is possible to hook the valve onto the rack the wrong way. Instead of shutting off by pulling the rack to zero fuel, it pulls the rack to max fuel
One suggestion to verify you have it hooked correctly, is to use a mity-vac and pull vacuum on the replacement actuator. Make sure the "stop" lever moves like it should, before starting the engine.
Also before starting the engine for the first time, have a plan for how to smother the air supply with a piece of plywood, etc. just in case. Do not use your hand!