This question gets asked a lot on a couple motor repair forums I am on. Short answer, no, it usually isn't practical.
Long answer. Yes, it can be done. The biggest question is usually with converting 9.9 to 15. For a bit, certain OMC 9.9s were detuned 15s, and once someone figured that out, everyone wants to do it to any ole' outboard.
But, on most motors, it isn't cost effective. Often, the powerhead may have the same bore and stroke, but that could be the end of all similarities. Now, I am not at all familiar with Yammies (nor do I see a need to become familiar with them any time soon :wink: ) but I think I know where you could get the info on the practicality of doing this. Check your PMs. But, the chances are, you will be better off trading up to a larger motor.