1978 v20 Mercruiser 470 possible replacement needed please advise

[surchin]

Hi all Scott in Santa Barbara here. Well got the boat in shop and after new fuel new carb petronix ignition it appears my 1 and 2 cylinders are not firing. That's as far as we have gone don't know about compression or maybe bad wire or plug , the easy stuff,. But I am thinking it may be terminal I am expecting worse . I love my boat but what would be best way to replace a 470? I don't have time or place to do work I'll be paying for labor. Not sure what I'm going to do- is there a value to an otherwise good condition v20?