You might be able to used compressed air. But you would definitely need to have the bottom pan off and also the Alternator side cover and get lucky. This also happened to me during reassembly but I had the engine on a bench. I tried the same approaches as you. bore scope, compressed air and magnets. Ultimately I had to bring the motor onto the basement floor that has a man cave carpet and rolled the motor all around. Upside down, left, right, wiggle wiggle wiggle, roll, twist, listen out for the bolt movement and it will eventually ended up in the bottom of the pan. Don't forget to take out your valve shims and buckets as they will fall out if you are working on the heads. Also, leave one bolt in the pan and leave the pan on. The pick up shaft protrudes really low and will create an unwanted pivot point and can also chip or introduce particles in the motor if exposed. Good Luck