I've had two SAE-300 units with what I had always been told were 6 cyl Continental's. Magneto, updraft carb, a little touchy but if everything is set right they run so sweet even a monkey can certify with them. Both in mid 1950's according to the SN. Local welding supply has one for sale right now for $2000. It is very clean and runs great.
Never seen one with a v-8. Unless someone else here knows more about them, you can get info from Lincoln, especially if you can get the SN#.
Have a great day. Brent
They used to sell just the generator part for people to make their own machine from. Any old auto or tractor engine could be used. I have a book from Hobart on building Your own, I am sure Lincoln had one too. I have seen the generator only in the Lincoln Owners Manuals listing on their website.