Welcome to the forum!!
So you want to replower a Atomic 4 30 hp gasoline engine with a Universal M-35 30 hp diesel engine? Some considerations include dimensions, including footprint, exhaust system, transmission, ignition system, etc. Suggest discussing the conversion with your local marine engine mechanic to assess the work scope and costs. Once you do, please let the forum know the feasibility, work scope and cost so others can learn form your experience. I am sure the forum will plenty lots of sound advise, besides asking why you plan a repower and if you've considered rebuilding the Atomic 4 engine, but I will leave that discussion to others.