Basically your options are to get a second undergraduate degree or just take classes to fill in holes in your knowledge. But based on what you have said, you have many holes to fill so in the end you probably would end up with a second degree either way. I think the only way that not getting a second degree would be advantageous is if you were working part-time, due to time constraints.
How long it would take depends on full-time vs part-time, how many hours from your previous degree you would be able to transfer/count towards second degree, and how many classes you can take in a semester. I would estimate 2-6 years.