I will call LBS when they open, if it is possible to order a left arm I expect it would be the best solution. I guess the arms are alu, because it says the spindle is cromoly. I forgot to say, this is the 2nd incident with the left arm coming loose. The first time I cleaned all the mating splines as well as I could, added a little fresh grease, and put it back together.
Instructions say drive side crank-arm pinch bolt gets 37 lb-ft, and non-drive-side gets only 7 lb-ft (!!) That's not much at all, I'm sure I got more than that with the multitool, is my problem maybe from overtightening? (Not that at this point I could solve the issue by putting it back on looser)