THere is a fellow , MDS, who frequents Boatdiesel and advertises there.
He is a frequent contributor and knowledgeable about those engines.
Before giving up on the Detroits look him up on B.D. and contact him.
His site is Marine Diesel Service. He is out of state for you but regardless contact him as he has helped many.
An oil leak? Where? Many oil leaks are valve cover gaskets, pan gaskets, front crank seals, maybe a rear crank seal. Lot less money to replace gaskets than swap engines.
You will be surprised at how much work and expense there is to a so called simple swap especially when the engines are different.
You might even ask MDS about your thoughts as a consultant.
I have an old Cummins, older than yours, and it too leaks oil but I am not thinking about changing untill the actual engine shows signs of wearout.
IMHO, Oil leaks are a nuisance, not a cause for a swap.
Due to injectors I changed the valve cover gaskets and the new ones slowed a lot of oil.