Once the bike has been ridden with the crank arm loose the inside taper has likely been damaged and the arm will need to be replaced. It is highly unlikely that you will be able to file it back into the proper shape, nor will the bolt hold it no matter how tight it is torqued.
Replacement arms are available from Amazon, you will need to know the length, which should be cast into the inner side of the crank, it will be a number near or equal to 170(mm). You will also need to determine if it is a "square" or "diamond" orientation. https://smile.amazon.com/s?k=bike+cr...f=nb_sb_noss_1 This listing has examples of diamond and hole orientation: https://smile.amazon.com/Epessa-Crank-Left-Diamond-170mm/dp/B07K7579HK
You will also need a wrench to swap your pedal, the left pedal is left-hand threadede. Since you are a bicycle messenger it would be wise to invest in a basic set of tools to use in maintaining your bike; your crank failure is a result of not having the securing bolt kept tight; pedals are also susceptibel to damage from being loose.
Edit: If you have a Hollowtech crank instead of square taper these will not work.