I highly doubt that the government of Japan will allow the boxing match in Tokyo. As per
https://covid19japan.com/, Tokyo has the highest cases of Covid-19, so it's not likely that they will simply go there as venue alternative.
Aside from that, the entire Japan is being put under a state of emergency. This is in place until May 6. In line with this, the government is urging people to remain in their homes. So I guess this match will be put off indefinitely pending the stability of the situation. After all, both Japan and the US are suffering from the blow dealt by COVID-19.