Originally Posted by
Crankycrank
Looking at how much aluminum is missing a helicoil may not be a 100% guaranteed fix. Something like a threadsert, (see below), might be better but as you've seen this can get expensive for a proper reverse threaded part even when doing it yourself and drilling out the hole it has to be exactly perpendicular to the arm which can be a little tricky in this situation. Personally, would just look for a used replacement arm on ebay or from Co-Ops or you might find a new one for cheap somewhere.
They make inserts specifically for bicycles that are more akin to a "Timesert", which is an insert that has the same nominal thread pitch on two sides of a wall--it's the thinnest possible piece for a full-strength repair, which you don't get literally anywhere else. As was indicated elsewhere, removing more material than necessary may not be a good idea.
As Andy indicated, no fancy mechanical machinery necessary: the bicycle industry solved this particular problem many years ago with Eldi's reaming tool/insert set. It is a pair of oversized pedal taps, with a tapered reamer at the nose. Provided the arm in question is not completely knackered, no special tooling or experience (other than basic mechanical fabrication skills) are necessary. The tapered nose makes it almost impossible to tap into the arm off-center. Unior's current taps have a longer taper, which makes it easier to start in a "just barely stripped" hole.
That said, the OP's crank is, well, likely too knackered to use. If there is any unsupported part of the threaded axle between the end and the face, it will try to rock, and there is nothing you can do to stop it. If your repair cannot restore that part, you cannot fix it.