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a mechanical p pumpped cummins would be the cheapest. The had the 4bt in bread delivery trucks with th400's so you can snag the adpter plates from a 4bt which is the same as 6bt minus 2 cylinders. Since it's mechanically injected no computers needed. That would be the cheapest for a swap. Even a little 4bt in marine trim makes somewhere in the hood of 210 hp. and gobs or torque.