Although the adjuster is convenient, you really don't need it.

I agree that the long bolt is flimsy -- it seems to get bent when you adjust it or maybe just from the forces on it -- especially if you rely on it to maintain your belt tension -- and the bending weakens it so it eventually snaps off. My personal solution to this is to not use it continuously.

What I mean (viz., "continuously) is that I only use it when I'm adjusting the tension on newly installed belts. I believe that the adjuster is only meant to initially adjust the position of the alternator, not to be relied on to maintain belt tension day in and day out.

So, to hold that tension, instead of just leaving that adjuster as it is, I tighten the holding nut on the sliding bracket (I hope you know what I'm talking about). Then I turn that flimsy adjusting bolt off a few turns so that there's actually a gap between that bolt and the bracket, so that there's absolutely no forces on it (i.e., it's floating free). After I've started doing that, I never encountered a broken adjuster again. Meanwhile the belts keep their proper tension because of the tightened nut.

Anyway, to get to the point of this replies subject line, you only need a crowbar of some sort, maybe a long and hefty screwdriver, with which you can pry the alternator's body (be careful of electrical connections) to get the necessary belt tension, and then tighten the aforementioned nut.

Hope this helps.