There is a circuit driving two steppers with a micro plus a gate array.

The pins used to activate the functions I need, belong to both. I will replace them with just a micro programed by me.

I do not want to do any "surgery" in the circuit other than desoldering them and auxiliary ICs not needed..

My doubt is what could be the best way to fit the micro to the board.

Just in case you ask, no way to solder it in the place left free by the other two chips.

Ideas anyone?

 

That description is awfully vague as to things like how much space is available. You might be able to make a kind of daughterboard to hold the new microprocessor and simply use wires to jump the power and control signals.

I would bet you will be wishing you had a completely new layout before long.