I bet I have that comic somewhere! Do you have it? That would be a prime target for vectorizing. That would make it scalable and clear. Unfortunately, my vector skills are lacking.
Did you make changes to the original when you made your poster that would like to keep? Or would you prefer to have a copy of the original comic cover? I say this, because it would probably be easier to reproduce from a smaller size like a comic cover than from something as large as a poster. I don't want to even think about how expensive it would be to get something that size scanned.