But, I not there
I not sure on this. Most you mention is not hard, but, you not need some flabbergaster to rip you off either. A good machine shop can fix your engine. This I know! But, as you ask, who good? I not ever go to a engine rebuilder place that mainly talk basically car engine stuff and can not tell me what engine a 120 inboard-outboard Mercury is. I have a machinist that do anything I need. I have known this man from when I was 16. He taught me how to put rings on a piston by hand and no fancy tools required. He told me what engine was in my boat from numbers on the block. He find parts that many will say do not exist (Not superman ....... He the machinist!). I not need him much today, but, usually have him Re-work heads (I can do this) and boil my block out (I not have the equipment). Well, his son-in-law now. But, his expertise is within the company. There probably someone there, I just could not honestly say who.
Maybe someone can help. I will do some networking on this and see if I learn anything worth mentioning.