Hi mate.. Helicoiling is for repairing damaged thread, not the best option for removing a snapped bolt.
1st thing, how/why did the bolt snap? if it snapped because of over tightening/faulty bolt, then as long as the thread isn't to badly stretched, the snapped off part should come out fairly easily.
If it is corroded in there and snapped trying to loosen it, then it will go hard.
So take the wheel off to give good access, I presume you have the adapter plate off, and the bolt is snapped flush with the frame?
I have seen people get these out by hacksawing a few mm into the mount, through the bolt, and unscrew with a flat screwdriver.
if you decide to drill the bolt, go to the hardware store and get the smallest "Easy-Out" or similar, and the appropriate size drill bit, and attempt to get it out this way.
If you have to drill the bolt out entirely and helicoil it, it is very hard to keep the drill centered on the hard bolt and not dig into the soft alloy casting. basically you drill the whole out to the size of the bolt exactly, tap a new larger thread, then fit a helicoil to reduce the thread size.. not really helpfull unless you can get the bolt out...
If it goes badly, it would probably be easiest to weld up the whole, then drill and tap a new thread in the correct position.
the hacksaw trick would be my choice, but i'm dodgy..lol