This is how I went about fixing the broken starter idler boss on my bike. It seems to be a common problem and there are sure to be more, so this may help someone in future. This is as yet untested but I am pretty confident. On early bikes with the unmodified cases, the boss is so weak it's a time bomb.
First the broken boss. I fixed the two broken parts back onto the rest of the boss using JB weld. I made sure to remove the powder coat and roughen everything up good to provide a key. Next I took an off-cut of handlebar and JB Welded that over the boss. The tube is maybe 1mm bigger inside diameter than the outer of the boss.
The boss extends back beyond the drilling that accepts the idler shaft. I cut the tube 17mm long to overlap the weak point. Next it had to be left 24hrs to cure.
The weakest link here is the JB Weld, but provided it is used properly it should be fine, the product claims to have a tensile strength of 3900+ psi, and to tolerate a constant temperature of 500f and immersion in oil. I am pretty confident that this repair would work on it's own, but I am not relying on it. The main purpose of this part was to rebuild the boss for the second part.
This fix originated with site member Marc Hanna. He reports that it is holding up well.. It involves using the idler shaft as a guide, and drilling into the crankcase wall. I drilled in 1/2" without breaking through to the other side. Marc suggests 3/8".
The drill bit is then inserted into the drilled hole, shank end first, and then the shaft is fitted over it. The drill bit is then cut flush with the end of the shaft, and the end bevelled to avoid any sharp edges. This is what it looks like when done. The drill bit will take most of the load and restrict lateral movement of the shaft.
As I said before, it's untested but I am hopeful that it will be a permanent fix. If it does not work out, then at least I am no worse off than before. I will report back if it fails.