Hello Pabst
The rust spot looks bad on the surface but might not be as bad as you think. Carefully sand the rusted area - just a little at a time. You could use a bit of Naval Jelly or other rust remover but just in real small amounts and only on the affected area. Try a small amount of that in conjunction with fine sanding. Use a magnifying glass to look for deep pits. As long as the tube has not rusted through, you can get the rust off and then use some filler to patch the area. The tube should be about 0.030" thick in that area. It may have a slight loss of stiffness at the affected area, but probably not enough to notice or cause worry for the long term. the important thing is to take care of it now since chrome-moly is actually less corrosion resistant than plain carbon steel and it will only get worse.
If you have access to an ultrasonic thickness tester, that can tell you a lot about how much good metal has been lost in that area.
Best of luck, the bike looks pretty good otherwise
Scott