"I'm wondering if it's worth while to install an inboard engine in an O'Day 25 that wasn't designed for it, Rod."
Well, Joe, the inboard WAS an option on the 25 right from the 1975 introduction of the 25. However, the 25 owner across the pond asked for info on installing an inboard in place of his outboard...... and I provided some info. In truth though......perhaps I left out the most important information! It is possible to repower a 25 with an inboard Diesel, and it won't affect the trim that much (remember, inboard was a factory option) or at least not enough to worry about. What I absent-mindedly left out was the biggest downside to this repower.....$$$$$. Assuming a brand-new Diesel and all running gear, installed by a professional......figure in round numbers to spend at least $5K, most likely much more. So, unless he LOVES this particular O'DAY 25 and will sail her for a good long time (does anyone other than you and I keep a boat that long??) this repower will double the value of the boat, but not really increase the resale value, in other words, I doubt one could expect to really see much payback at resale time. As you say, better to find another 25 (or 26) that already has hte inboard Diesel installed from the factory.
An inboard DOES have advantages: with prop located well below boat, it won't come out of the water in a sea (well ,I guess it could, but you'd have other worries then?), prop is ahead of rudder for better flow of water over rudder at slow speeds, Diesel vs gas.... some consider Diesel safer due to less explosive fuel (but with an outboard and keeping fuel tank in cockpit or better yet vented locker like on 1979-83 25 and your 222, as long as scuppers are not leaking.... no gas or fumes should get below anyway), thrust is on center, not offset to one side like outboard would be on 25.
Inboard does have disadvantages too, besides cost..... most inboards are 4-stroke and so unlike our outboards (2-stroke models), an inboard can only be run when the boat is relatively level (up to a minimal heel angle) otherwise the lube oil in the crankcase may not reach vital parts, I never would have thought of that.... but several friends with inboards have mentioned it. I'm not sure why, but this does not seem to affect 4-stroke outboards. An inboard will increase drag, you can't retract the prop while sailing (well, again...... that can be done, but unless you have a top-notch racing boat...... it would be pretty pricey! I have a picture of a racer hauled out down in Newport that does have a retractable prop and shaft!)
So, anyway........ he asked, I provided info. Would I recommend repowering with an inboard?? Well, maybe not after considering cost. But, it is his boat, and the job is possible!
The 25 below is a 1983 model with a Diesel inboard, not the best pic....but she trims pretty level.