My boat (bought second hand a year ago, little history except docs and receipts left aboard) has a Lombardini LDW 1404 engine. Clock now shows just over 14,000 hours. Apart from problems with the fuel supply (not specific to the engine) this engine has run consistently for the year I've had it. I had the cam belt replaced last winter, for which total cost was about £250 - for the kit, including tensioner, plus about an hour of engineer time. The replacement was slightly but not excessively fiddly - engineer was in and out in an hour, seemed perfectly simple.
Having no rev counter I have no idea whether the engine ever delivers its nominal 3,600 rpm. I just run it through 4 sweet spots (avoiding resonances with other bits of the boat) - idle, first sweet drive spot, 'upped' drive, and 'flat out'! I find the engine somewhat noisy and rattly, but got used to it. Conversations at the stern require slightly raised voices! From the bank, it seems no noisier than other narrowboats to me and others. On the whole I feel it's reliable. Anyhow, it took me safely from Bristol-Portishead-Sharpness this summer, although I had to run her for about 3 hours at 90%+ power to keep up with the piloted boat in my 2-boat convoy. On that voyage, I ran some hot water off in the galley at about the halfway mark; by the time we reached Sharpness the engine was hot but not steaming, and there was no significant coolant loss.
Lombardini on a narrowboat - a bit of a rarity perhaps, but so far I have no complaints