I'm doing a re-power now, having had to replace leaking fuel tanks. I was going to put in the Cummins 6BT's. Price was $16,000 each, plus install. They are probably the best option if you aren't doing a lot of hours. They are labelled as 'reman' but in fact are brand new engines.
Make sure that you check gear ratio and prop sizes, or you might need new gears as well.
But I ended up with the John Deere 6068 TFM 75 engines, Tier 2. They were around $25,000 each, plus install. Big difference, but the JD's will be up to 1 gph more fuel efficient at low rpm, and I'm trying to maximise range. With larger tanks, some bladders and the JD's I should be able to get to Hawaii from US west coast. Then its relatively short hops back to Australia. Given the shipping quotes ranged from $55,000 to $100,000 I should breakeven or better. And enjoy the trip.