FWIW you probably don't need a USB serial port, or a RS232 converter at all; you could just pick the TTL serial signals off the commodore port and plug them into the TTL serial port on the Raspberry Pi 23- or 40-pin expansion slot (pins 8, 9, and 10 are TX, ground, and RX.). Remove the tty on /dev/ttyAMA0 from the init config and you can use /dev/ttyAMA0 as your built-in Pi serial port.