engine
I put a new Yanmar 18 HP in an O'Day 28 that previously had a Universal 5411 (11HP) about seven years ago. No regrets, it worked out great. It cost me about $6000 plus a little but I also had to get a new prop and replaced the exhaust system too. Very happy with the results but I did upsize to the next boat a couple of years later, you know, 2 footitis or actually 5 footitis.
I had to modify the engine mount just a little bit by cutting a small piece of it out. I used a chain fall to get the old engine out and lower the new one in. Otherwise, the new engine fit right in with very few problems.
Best of luck,
Chuck