Hold on .. are we certain here that the thread has already been helicoiled before ?
Its quite possible to pull out the entire original aluminium thread with the bolt, and it will look very similar to a helicoil insert.
I'm more inclined to think that this is what has happened.
Either way, I would still start from the beginning of the helicoil process and run the tap down the hole, even if its only to clean up the threads before fitting the insert.
But I strongly suspect that you will have to actually cut the new thread for the helicoil.
There may even be further complications if the clearance diameter of the plain half of the hole is too small to pass the helicoil tap through.
You could of course tap it from the other end but then you significantly increase the chances of the tap not going in square, as it is no longer guided by the plain section.
If it comes to threadcutting, use paraffin as a cutting fluid/lubricant on the tap, cut the chips off small and blow them clear to avoid them jamming in the new threads and ripping them.
Remember, you have one shot to do it right, or else its a new fork bottom.
If I were doing it from scratch I'd strongly consider doing a practice run on a bit of scrap aluminium first.
Either that or find a local garage that will do the work (and take the risk) for you.
When its all done and you're retightening the two bolts, nip them up very gently and gradually, moving repeatedly from one to the other as you gradually approach the clamp torque on the spindle, and remember they only need a very low torque.
Use lube on the bolts, not Loctite .. black moly grease ideally or fine grade copperslip.
Worst case scenario, as a super-bodge you could put a nut on the back.
You could at least ride it to the garage like that.