Hmmm. Interesting. "I bought the motor from the brother of the original owner"....
Was he Australian?
At any rate, trying to do what you are wanting to do, conversion wise,
will be fraught with unknowns and expense even if it were an American market Honda.
My advice, to pretty much anyone considering extensively converting an outboard in this manner, is don't do it. If it's not what you want, sell it and find the one you need.
The used 10+ year old Honda outboard market is pretty robust and should offer many good, well maintained, outboards that will likely cost you LESS than the ultimate successful conversion.
Not what you wanted to "hear"?
I get it. Sometimes a guy just wants to do what he wants to do.
In that case, about all I can offer is that I would GUESS that the Aussie and American models are close enough to each other that you can source the parts from this continent. After all, I see Australian owners referring to buying parts from boats.net (and complaining about the shipping charges) quite often.
Also, you should open another thread about measuring the case for X and XL or ???. Hopefully you will get some ACCURATE direction about measuring from someone that's actually done it. I haven't.
On most I believe the extensions go in 5 inch increments. IE:
Std =20"
L. =25"
XL =30"
XXL=35"
I believe that the XL and the XXL extensions simply use the std. OR the L extension, respectively, with 5" "spacers" or inserts to achieve the desired length.
That may or may not be totally accurate.
And, I don't know about exactly how to place the tape measure to discern the exact difference.
I would also wonder about the diameter differences that may be present in the longer main shafts. That could be an issue to consider.
Good luck.