Would the head gasket still affect the car from starting even if I have a new motor?

Today I had a mechanic look at my car and he told me that the head gasket was blown and that the antifreeze was leaking out into the oil. He then told me to go find a motor so he could replace the old one and he also told me to go buy a new thermostat, oil filter, spark plugs and spark plug wires (Because the spark plugs in the car has no wires attached to them). Afterwards I asked, "Why not place the head gasket also? Wouldn't the head gasket still be an issue for the car even if I bought a new/used motor?" What would be the issue?