Lotsa good advice here and also some biased opinions and incorrect information. I often run across N-owners who want to do an EI switch because they are having problems with their current setup and A) the tractor won't start, or B) the tractor won't charge, or C) the tractor runs for a while then quits. The thinking is to just go to a an EI system and all problems will be eliminated. Here's what I think...
First, how often do you use your tractor? If it is not a worker, don't invest in EI. You say you have a 1940 9N, Those are becoming more and more collectors items. You may have some rare parts on yours that would make a full restoration project more worth the investment.
Secondly, as mentioned, if you have a working tractor, I would go to a 12V system at least, maybe EI if I used it everyday. FWIW I have an early '48 8N still original 6V system that I maintain correctly, and is a working tractor, but I will get her gussied up on occasion and take her out to local shows.
Next, if EI is the way you want to go, heed the advice here and make sure it is already in good running condition before you start converting. As I said earlier, I see many fellas with tractor issues figurin' a 12V conversion or EI conversion will fix all issues when in reality the true root cause of their problems lies elsewhere like in fuel and carb system, charging system, bad wiring, bad switch, or ignition -no spark.
Lastly, I have never done an EI conversion so I cannot say whether the investment is worth it or not, but if you are having starting issues now, you need to do a logical systematic problem solving procedure to dig to the root cause.
Most starter/alternator shops will bench test your components for FREE. Take your generator, ammeter. coil, battery, and voltage regulator or cutout to them and start there. Polarizing your generator whenever the battery has been disconnected then reconnected is mandatory. Always hook up the battery last after mucking with electronics. There are procedures you can do yourself to test components as well.
Hope this helps