140 mercruser engine c.i.d. identificafion by engine serial number

This fall broken connecting rod came through the port side of my 140 hp Mercruiser 181 c.i.d. engine block. I have purchased another used engine and, it is supposed to be a 140 hp 181 c.i.d. Both are in the mid 70's. I am in question about the c.i.d. of this new used engine. The heads are not the same, the heads look the same except for the size of the intake and exhaust ports, and the spark plugs.My original head takes tapered seat spark plugs and the new head takes gasketed spark plugs. My Question is how can I tell the C.I.D. of the replacement engine, without removing he head. The Mercruiser I.D. tag is missing on the replacement engine. The only thing which I have to refer to is the engine serial number stamped into the starboard side of the engine block above the fuel pump behind the distributor. M10157RA I am concerned that I may have purchased a 120 hp 153 C.I.D. engine, I believe that both engines look alike. Can anyone please help?