The BF2803 is a wonderful engine. It is smooth running, quiet and lasts a very long time if maintained properly. Kubota stopped production of it some years ago because it was too expensive to produce.
This engine is effectively a 5 cylinder version of the V2203 and all parts are available for both the engine and for the marinising parts. Here you will find the Beta Marine factory in England very helpfull.
They can supply workshop manuals, parts books and an owners/operators manual. I suggest that you email David Morris and he can connect you with the correct person who can supply all that you want.
His email address is davidmorris@betamarine.co.uk
There is also a Beta Marine distributor in France.