Boat Diesel.com is often recommended as a good source for information. I spent the dollars and joined but was very disappointed to find as you did that there is little or no information on the 120 lehman. The specs sheets are totally void of information. The discussion lists seldom have anything to do with these dependable old diesels. I let my subscription lapse as I received virtually no good information from that site.
If you have other brands of engine your experience may vary, but for me and my 120 Lehman they were a total bust. (Oh, and there is no way to get your money back once you find this out)
Marins recommendation of the GB site is spot on. It seems like a majority of GB's came with Lehmans up until the "go fast" era. The owners there are very knowledgable and ready to help. Even if you do not own a GB the knowledge there is top notch. I fully intended to be a GB owner and started my pre-purchase research there. I never left, even though I wound up buying a different brand.
American Diesel is indeed the place to buy pieces or get help with a tricky problem. Every time I call, Bob personally answers the phone and talks to me as if we've been having coffee together before work every morning for the last 30 years. There is no better parts service available, IMHO.
Ken Buck