substitutes for irf640/9640?

You should try to stick to original engineering and parts unless you want to deal with unpredictable results.
Plus 640 and 9640 are fairly cheap everywhere why ask for more trouble by down grading to a lower voltage part.
The maker felt it needed the higher breakdown voltage device in the first place perhaps their reasoning was valid.

Sometimes a simple repair gets hard when you put in lower grade parts:)