You have to understand that the engine SPEC number is what tells the story. Every engine manufacturer made basic engines and then worked with the end-user companies to give them what they wanted. That means things such as
- starter or no starter
- starter position
- fuel pump or no fuel pump
- exhaust or no exhaust
- deep, shallow or standard oil pans
- crankshaft stub length
- crankshaft stub diameter
- crankshaft stub style - tapered, straight, keyed, splined ????
- alternator output
- regulator included or not
There's probably more variable than that but.......you get the idea. Get the spec of the 990 and give it to Boomer. He will tell you how that engine was originally built and why you have to do to make it work.