楼主不太明白被动结构的特点吧?
for当然不能省略,否则无法连接后面的imported ones.
此句还原成主动句如：People are substituting home-made parts for imported ones.
substitute此处的确做及物动词,构成固定短语substitute A for B,意思是“用A来代替/替换B”.
变成被动即：Home-made parts are being substituted for imported ones.
substitute的宾语是home-made parts,其在被动句句中的主语.
如果你满意我的答案,这是对我们答题者劳动的尊重,多谢!