Are there any currently available antiviral drugs that could reverse virus-caused dementia, like ivermectin or hydroxychloroquine?

Both of these drugs are capable of bringing about a clinical improvement in people with dementia, regardless of the diagnosed form. While they are not necessarily a perfect answer, they are inexpensive and readily available and safe to use chronically, and that will not be easily achieved exploring novel chemical approaches. In the meantime, anything that can help is well worth employing, given the bleak outlook for people once symptoms of dementia appear because it is almost always progressive.