Hi,
I have some experience in this matter being somewhat of a mechanical
thug regarding cheap rusted bikes I was basically given in my youth.
You can hammer a cotter pin tight and it will stay put for a bit,
without a nut, but when it loosens it comes loose really quickly,
and the bike becomes rapidly completely unrideable.
Which will be going up a hill probably. Drilling out your cranks
to the more common size is an option. Filing is about the
angled face and enough thread sticking out for the nut.
rgds, sreten.