Ebay, craigslist, boatyards, forums like this, etc. are places to look for used diesels.
If it were me looking for a diesel I'd avoid brands like Volvo, Farymann and a few other odd brands as parts are either absurdly expensive or just not to be found.
What I've been doing my best to NOT tell you is that if you can cross reference which Kubota block is used in the 'A4 drop in replacement' engines, you might be able to find a cheap engine that came from a piece of used farm equipment, like a tractor.
Here is an sample of the cross referencing I am talking about
: Kubota / Universal Injector parts Numbers & Engine Block Equivalents
Once you find out the Kubota block # you then have to figure out which Kubota tractor it was installed in to find parts for it.
From a purely monetary perspective I'd probably go for a re-built A4 from Moyer Marine for around $6K and call it done. If you are planning on motoring for days and days I'd lean more towards getting a diesel re-power.