There is a way to convert your Honda BF5A^la to a short shaft. You may not like the answer but it is a very expensive undertaking. From Honda , there is no kit available to do this but you can purchase all the seperate parts to do this.Basically it is just a matter of changing the midcase or the exhaust case and changing the shaft and water pickup tube.
I have done this before several times but have found it easier to just sell the 5 long shaft and buy a short shaft. Honda 5 is a sweet engine but I have found that the Tohatsu 5 is a much more balanced engine and priced nicer than the Honda. The Tohatsu actually weighs 5 pounds less than the Honda
Assuming that you are going to use the engine with a sailboat, why would you want to convert it to a short shaft. We've had both on a C22. You get lots more power, especially in reverse with the long shaft. Even a long shaft is problematic for a sailboat in choppy water as the prop pops out of the water occasionally, over reving the engine and loosing power. If on the other hand, you want to use the engine on a dinghy, jon boat, or such, what would be your objection to the long shaft? Conversion can be done I'm sure, but you're going to spend a lot of money.
i had a long shaft converted to a short shaft and the price was not terrible but i had taken it to a shop and told them i was in no hurry. it took a couple of months but eventually they came up with the necessary parts from DEAD motors. of course this is only good if you can wait.