If one were wanting to experiment with the serial interface and ftdi cable, what are the suggested crystal and cap values? The datasheet offers a number of values with differing serial error rates.

OR, at slow speeds like 2400 baud, is the internal clock good enough?

One thing to be sure and note is that the Larson Scanner uses the low-power version, the ATtiny2313V, so that it can keep working even when the voltage falls very low. The ATtiny2313V is limited to 10 MHz, so if you do use a crystal and cap (or ceramic resonator) don't try the 16 MHz one.

8 MHz is a popular choice for these chips.

Now, I do have reason to suspect that the AVR internal oscillator is sufficient for basic serial communication. The original Lilypad Arduino used the internal oscillator, set to 8 MHz, for operation, and supported serial programming, so it's certainly worth trying... and it's likely that you'll succeed if your clock rate is low enough. (The internal oscillator is set to 512 kHz in the default program-- you'll almost certainly want to change that.)

Also, as hinted at in this discussion, you might even consider mucking with the clock calibration bits to achieve slightly better accuracy.

Glad you pointed out that the supplied chip is the V version, I had over looked that. The two xtal choices seem to be 3.6864 and 11.059.

I'd suggest a FOXSLF/080-20 crystal, with two 18pf - 20 pf caps.