Just an update. I have gotten a Stihl straight shaft and it went on with very little effort. I now have a Stihl fs47.5 trimmer [(fs40+fs55)/2]. The replacement carb on the old Shindawa didn't work out.
Here lately I have been buying the cheap FS38 trimmers, selling everything from the shaft down on e-bay, and slapping the powerhead on what used to be fs90/91 trimmers, and it works good. It even runs the chainsaw head attachment well.