That pin is called a woodruff key. The buried part is round. You can buy them at a parts store or a hardware store. Usually you can get a chisel under one end and drive it out.
Cranks can be welded, but it's not cheap. Running with a damaged balancer scares me. It can easily damage the crank, bending it or causing fatigue, shortening its life before it cracks and breaks. I would be looking for another crank.
Look closely at the main bearings. You will be checking for wear patterns that are skewed. If there are any, the crank is definitely bent, and I would ask about having the mains line bored, to be sure everything is straight.
Also, when you take the block in for machining, tell them about the balancer. Be sure they magnaflux the block before any machine work is done. This a an inspection procedure done to check for cracks.
Hopefully Chiphead will see this and chime in. He has far more experience in dealing with screwed up engines than anyone else here.
