Search around for the service manuals, you can find the way to fix the rangefinder. It's not hard.
I often take a single piece of cardboard, paint the back black, and make it slightly larger than the hole in the front standard. It is held in by the lensboard sliders, and actually works. Not pretty, but it works, and if done right does not leak light.. You can also get lensboards off of ebay, depending on the model you have.
The lensboards, have no groove in them they held in strictly by the sliders on the front.
As said before, dropping the front allows you to do the other movements you desire.
That little lever engages a governor that changes the shutter speeds. Look in the little window with the speed in it (if it's a Pacemaker) and push the lever, and you will see a mask slide over to display a different set of numbers. This allows you to change between the high and low speeds (1/500 vs 1/1000)