outboard power for pocket trawlers
The current issue of "passage maker" has an article about someone who made a very similar conversion, on a 30ft boat with an inboard diesel to twin Mercury outboards. The work was done at Zimmerman Marine, those guys do quality work and from the photos looks like that was the case here, also.
The converted boat is more of a "down-east" than a trawler, but I don't see why you couldn't do something similar.
They wound up using gas outboards, but Mercury and several other brands make diesel OB's in a range up to 200HP, at least that I have seen, so you could go either way.
They did a lot of other mod's while they were at it, and the article didn't say how much the work cost, but makes for interesting reading, and may give you some good ideas.
By the way, Seaway is another builder with a nice line of "pocket trawlers" up to 27ft, all with OB's.