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Are you sure it's screwed? I was 5 miles from home once when my nds crank arm came loose. Had to use 2 alankeys side by side to tighten as I didn't have one big enough but couldn't get enough torque on it. Had to re-tighten every quarter mile or so but carried on regardless (desire to get home > cost).
Anyway, tightened up properly when I got home and never had another issue with them.
It's easy getting another non-drive cranks for a tenner, the hard part will be finding a 165mm one.
Go to any bike shop and ask if they have any 165mm left cranks arm, and perhaps worth asking them to ensure the bolt are correctly torqued onto the cranks.
It's also worth greasing the spinel a tiny bit (anti-seized) to ensure a good fit.