Need to ID my bike and a Left Cotter crank arm

Hi I recently came upon an old family bike which I want to make road worthy for my daughter. This is quite an old bike and all I know about it is that it was made in Poland.

I've been lucky enough to find tires for the original wheels, and a new seat but I've had no luck finding a left crank arm.

I've been told that I need a cotter cranck arm, which is a design and style not made or used in the past 20 years!

Any help as to where I can find parts for these bikes, and if anyone knows the type of bike this is (it might make looking for part easier) would be really appreciated.

Thank you.