outboard, no problem
We put a 9.9 honda 4 stroke longshaft (75 lbs)on our Cal27. Works like a charm. Original inboard was removed by previous owner. We used 1/2 inch ply to reinforce the transom and got the top-line Garrelick (eezy lift spring loaded model). In addition to that, we added 100# of sandbags in various holds to make up for the weight difference of the inboard. Made the boat a better, stiffer sailor and just as fast. I'd use a 15hp for the 29, as the displacement is about 3000# more than the 27. I'll look for a pic and post later. Good luck.
p.s.- we just moved up to a 1979 CAL2/29 with a Faryman 12hp. 2 weeks in we're loving it. Much more boat and plenty headroom.