You have to keep in mind that the main issue with brackets is generally not the weight of the unit; the OEM brackets on the Oday trailer sailers were rated to hold bigger engines than a 9.9. The big deal is repowering with a four stroke engine, as opposed to the two strokes that were universal when these boats were built. Four strokes generate more torque than their two stroke cousins. The older brackets can probably handle the weight of the new four strokes no problem; its the torque that will be an issue. If your bracket is not specifically rated to handle a four stroke engine, you are taking a chance on having your new outboard ripping the bracket out of your transom. You can buy a new four stroke rated bracket, but that will probably require you to also build a new "spacer" block (that angled wedge of fiberglass inbetween the transom and bracket) too. The footprint of the four stroke rated brackets is wider than your spacer block. There are a number of threads on this site where several posters describe how they did just that.
This is not an unusual problem; ever since they stopped making small two stroke engines, lots of sailers are having these same issues. When I repowered about five years ago, I still had the option of buying a two stroke. I did just that in order to avoid the bracket/spacer issues.
