Electrical problem

The car is a 2000 CLK430. It has had some parts removed and replaced with parts from other cars. Slightly stripped. The problem now is the ignition switch won't work. It had a key in it when I got the car but it wouldn't turn. I am fairly sure it is the wrong key. To make it worse the dash has been switched also so the ignition switch may be from another car. I could order a new key using the vin number but I am not sure if it would work in the ignition switch as it may be wrong also. New one is around $350.00. I bought a key and switch from a scrap yard and it will work the door locks but nothing else. It isn't sending the correct message to the engine computer it seems. Any ideas? Jerry