Hi Ralphv,
I was in the same situation last year looking for replacement turbos. I have a pair of 1998 6cta8.3 Diamonds with about 1700hrs. The turbo on one engine developed an exhaust leak; the bottom 2 bolts on the turbo to exhaust elbow rotted away and the housing was too trashed to have machined. It was impossible to source new turbos. Sea Board Marine didn't have one, Cummins was back ordered at least 9 months and everywhere else was waiting for their stock to be replenished with no indication when they'd have one. I found a Diamond Diesel out of Oalkand, CA that had a new housing and cartridge and the could rebuild it. I went that route at a cost of $2700.00 with all gaskets required to reinstall and the turn around was fast. I was back on the water within a week. At the same time I placed an order with MD Distributors
https://www.mddistributorsstore.com/holset/3538624h/new-water-cooled-turbocharger for a new Holset to replace the other side at a cost of $1730.00. They had been back ordered but I was on the top of the list when they became available. It ended up coming in a month later and I proactively replaced the remaining side.
I added manifold side, pressure boost gauges to both engines so i could monitor and compare the too. The difference between the old turbos and the new turbos is amazing.
I see the price on MD Distributors website went up a about $100.00; but it is still a bargain compared to buying thru Cummins. I can tell you first hand that the Holset turbo is exactly as the original.