It seems like it would be pretty easy to just have a step down transformer from 120VAC to 9VAC. Also wire up the 9VAC to a bridge rectifier to get 9VDC to then

It seems like it would be pretty easy to just have a step down transformer from 120VAC to 9VAC. Also wire up the 9VAC to a bridge rectifier to get 9VDC to then convert down to 5VDC with an LDO or buck regulator of proper current rating. It wouldn't be the cleanest power supply but the C64 board should have proper filtering for each rail.