OB forever!
I had to replace my 12 year old OB about 18 months ago. A brand new 15HP Johnson 2-stroke (a 4-Strk won't fit in the lazarette w/o some semi-serious surgery) long-shaft, electric start w/hi-thrust 4 blade prop cost about $2,200, and took 2 days to install, including the remote control. Lots of power, maybe even more than needed. Takes about an hour to remove to take to the dealer for service, if need be. Another thing that is an advantage to OB's that I've never seen mentioned is the ease of clearing the prop of line or weed you might pickup. You don't have to swim (or haulout!). Just tilt up the OB, lean over the transom, and pull or cut it loose. My 27 is a 1974 std. rig with tiller. In racing, I'm passed all the time by tall rig guys in the light air of the Calif. Delta (8-12 knots), but they can't keep up in the heavy air (15-30 knots) of SF Bay. The tiller is cheaper, more responsive, easier to maintain, gives more cockpit and room below, and cheaper to rig an autopilot to. By '88 it's true most of the problem areas had been worked out, but retrofits to fix all those weaknesses are available from Catalina, most for under $100. About the only earlier weakness that isn't practical to fix is the mild-steel keelbolts. But if you're in freshwater, you get a good survey so you know what you have, keep the bilge dry (easy with an OB), and keep the bolts painted, you'll have no problem. And an early OB model is probably $5,000 or more cheaper than a later diesel.