<p>Thank you everyone for your helpful advice. </p>
<p>Yesterday I took the lens in to Mamiya's service centre in Tokyo and they were quite helpful. A technician sat down with me to test out all the lens functions (shutter speed, flash sync, etc.) and told me everything was in good working order. Then, he removed the rear element, and, unfortunately, determined that the fungus was <em>between</em> the lenses in the element. It would need to be sent out for disassembly and cleaning and that would cost around 160 dollars. Even he told me it probably wasn't worth it. He did however clean out the inside surface of the lens before reassembling it. </p>
<p>I asked if I could just replace the rear element entirely with one from another lens if I found one for cheap, and he said that I could, but that there were spacer rings inside that were all slightly different for each lens, and that if I put in a different element the focusing would be slightly off. Does this make sense? </p>
<p>He also told me that even though the fungus looks pretty bad, it won't affect image quality ... at least for the time being. I still haven't developed a roll so I'll have to wait and see if that's the case. I will post a follow-up when I do.</p>
<p>Thanks again! </p>