I agree with the thought that it will be a long time coming.
I have a fast RIB (Rigid Inflatable Boat) with a 140 hp outboard. There are 3 choices of engine for this type of boat
2 stroke outboard (least capital cost, best power/weight ratio, horrendous fuel/oil consumption)
4 stroke outboard (more expensive, better fuel economy)
inboard diesel (highest capital cost, lowest power/weight ratio MUCH lower fuel costs (runs on duty-free red diesel)
All of these options have been available for quite a few years now but the vast majority of owners still go for the 2 stroke outboard because at the average level of use the owner would not recoup the additional capital cost (and its lost ROI) out of the fuel savings.
It's a different story for commercial workhorses, where the rate of ustilisation is much higher.
I suspect the same issues will mitigate against re-engining.
Also of course a re-engine will have a lot of additional expense not incurred in a new build. (Throw away your engine mount, exhaust, ancillaries etc. etc. and replace them with new)
Mike