This is a tailor made question for diyaudio.com.
Also put the curcuit up. Sometimes substitutes depend on the circuit.
BF245A is a very very common fet. Its a usual substitute to replace an opamp in a DAC circuit in a cd player. Of course it also has a lot of other uses.
There is no tailor made question. both accounts belongs to me. I am trying to make a preamp appeared in Elektor some time ago. BF245 is not available in my area. as far as parameters go, BFW11 seems similar. I posted this in both forums expecting some expert advice. experience has no substitute, I think. shall post the circuit later.