Nice score. You are right that the black box is an RF modulator, the Vic 20 came with one. Earlier Vic 20's came with a 9VAC power supply like you have, later ones came with a C64 style PS. AFAIK all of them had only one joystick port and the games from the time only had single player mode.
Once you start loading programs from tape or floppy disk, things will get more complicated. There were several RAM expansion modules and programs written for one won't likely work on another. Like if you try to use an 8K game with a 16K expansion, probably no dice. The memory map changes with each expansion.
You might want to invest in this.
http://for8bits.com/jpg_fe3_case.php
