I know some will question this, but when done properly I've had good luck with epoxy. I'd clean the vicinity of the crack thoroughly and dry. Then use a small triangular or other file with a sharp edge, or a Dremel tool, to make shallow cross hatched cuts all around the crack; this provides micro-grooves for the epoxy to grip to. Mix a good deal of epoxy, preferably 2-tube style J-B Weld or other steel content type, and spread it to cover the crack on all sides if possible, engaging the file marks well. If you have easy access to the crack, the use of a 5-minute epoxy will be tricky but the fast set-up will limit bulging and dripping. Alternatively, you can apply 2 thinner coats of regular slow-set epoxy an hour apart.
Done right, it is not a weld, but rather just a bridging medium, a temporary one in this case where stress is likely. A Tektronix engineer cousin of mine, though, that owned a Marina on the Columbia, used epoxy to "weld" broken outboard motor driveshafts back together, believe it or not. I often use super glue prior to an epoxy job, to get the parts tightly together long enough to apply epoxy and get it set up, not as a permanent glue repair. Ambient temperature affects cure time, so don't be in a rush to leave if it's very cold out.
Another thing I've tried recently is new stuff I got at my local Ace in the glues dept. In your case, you'd simply super glue the arm back in place, then squeeze a good deal of super glue all over and around the crack line, similar to what you'd do with epoxy. Then you simply sprinkle/pour this new stuff (it's like fine powdery granules) onto the wet super glue. It hardens practically on contact, and the bond or "weld" is instantaneous. I learned after several tries to use only high-quality liquid super glues... the gel type doesn't react with the powder properly.
It's just a suggested option. If you have a drill and bits and other tools necessary, and can get at things adequately under there, by all means try the plate sandwich idea instead. You just don't want the bolts coming loose and parts (s***) "hitting the fan", so use lock washers. As others said, without being there to closely appraise the damaged area, it's hard for us to do more than guess at solutions. It's ultimately your call.
Joel