Cummins Mercruiser does all their testing and r&d here in Charleston, SC( I had a job offer from them right up till their budget got hacked) . They have a couple of 26 Bertrams running Merc Diesels. One is using the 1.6l Isuzu diesel(twin set up) with an Alpha drive, it does OK, but it needs a little more power, the weight and size of the 1.6 is comparable to the 3.0 gas engine. The other boat is using mercruisers marinization of the VM diesel rated at 150 hp, it performs better, but the size of the VM(and their track record) doesn't make it a good choice. I have never liked the VM, they are big(bigger than a small block), use odd ball electronics, and generally don't perform well. I recently saw the Yanmar test Bertram 20 with a single BMW(used in the euro X5) in front of Yanmars own drive, it was a nice set up, but I think its too much for an Alpha. If you plan on keeping your existing drive set up, you will need to go to an alpha lower unit at the minumum. You will probably have better availability of engine choices in Europe than we have here in the States. If you have the money, a Volvo D4 Sx combo would be nice. Also, check with Styer Diesels, they have a light weight Merc replacement. If you are bound to use your existing set up, you could probably marinize an Izusu engine, but parts are going to be high