You can use the TL084 but probably you'll get some "ticking" in the signal due to the LFO. The TL064 is used because it is a low-current device. For the rest they are pin compatible (see datasheets). So if you don't have a TL064, just pop in a TL074 or TL084. It'll work and just see how much "LFO ticking" you'll get.