Well, I myself find it will be easier to stay factory, than to do a mod like that. One thing I would like to do though, is convert my 1941 Johnson HD-20 tto the later simplex starter setup, like the Lightwin. I did this on my first Zephyr and it worked nice.
Yes, the 1949 Zephyr had starter parts interchange with the Big Twin. It was interchangeable with several Gale models. I don’t think the Johnson will be that easy – Good Luck. . . 😉