I guess you will just have to use the ground glass to focus for a while. The current focusing scale will not be right for the shorter lens, but you may be able to make a new one that will work. The movable part is what needs to be changed. I'd recommend a chunk of adhesive label on a piece of aluminum. Not very sturdy, but easy to do and easy to change. Do the math, make some pencil marks, then check it against the ground glass before making thing permanent. Keep the current scale, just in case. Even if you have a decent scale, the rangefinder will be off. The difference between 90 and 101 mm is enough so that you will need a different cam. Scan the web, you may find one or you may be able to make one. Unfortunately, attaching and adjusting the cam is a very difficult and frustrating process (I've done it - ouch). Maybe the best solution is to look for a 101 mm lens and shutter. They're pretty common on the used market. You can also find lens boards, so you will have two lenses for your camera.