I hope you have not purchased this boat yet. Re-powering this old girl could be expensive. Not just the cost of the engines, the gears, maybe shafts, controls, fuel lines and new water lines. Then the labor for r&r. And then, the fabrication of the engine beds for new mounting points. Hill time, Lift charges, ,,,
Those old Cummins may look tall, but maybe not as tall as the new straight 6 cylinder engines that usually would replace it. Your going need a good local guy to offer what can fit before you fall in love with any engine brand yet. Or down angle clutch brand yet also.
Rethink your purchase. Maybe easier (cheaper) to pick up a newer boat, with ready to go engines.
Probably think about how long you’re going to keep this ship vs. money in/out.
Or,, spend the bux on the Cummins. Parts are still out there. Maybe expensive. Maybe real expensive. May have to make or repair what you have. Get tight with a good machine shop.
This could be cheaper in the long run than a repower.
Keep in touch,