I recently emailed a senior professor in a neighboring state, that I've never met, with some comments on his recent paper and pointing to some of my papers with related results. It turns out he was aware of my research and invited me to give a talk at his University. I promptly agreed, but never heard back from him. Should I assume that he gave up on the invitation, or should I write back to him "inviting myself"? Several months have passed since our email exchange.
It's certainly reasonable to send another email just saying that you would really like to visit, and that you would like to take up the old invitation. By far the most likely thing is that the professor in question just forgot, and will appreciate being reminded.
Since several months have passed, it is more than reasonable for you to email the professor once again. I have been in a very similar situation, and the professor was very cordial in his response and apologized for his lapse in memory. Professors are usually quite busy so they will not be offended if you restart the email exchange. Don't worry one bit, you'll be just fine!