I'd like to build MXR's Blue Box and I don't have any of these on hand.

Since both of the 4013 flip-flops are being clocked together, at least in the sense of a ripple counter... you could use any 40xx series or 45xx series counter chip and use outputs A and B for the two outputs.

I think in general you'll have an easier time finding a 4013 than most CMOS counter chips, but if you have one on hand... go for it.

You could also use a 74C74 or another 74C series counter chip. Some examples would be 74C93, 74C161, and 74C393.

The most important thing is going to be the ability to take a +9V power supply which eliminates 74LS chips unless you want to add more junk and a regulator to deal with their 5 volt demands.

There are also the 74HC series chips, but I think they won't take any Vdd over +6 Volts so you would have to add a regulator.

Good luck,
-Peter