If you make a skin for the boston, there is NO guarantee it will work for the havoc. FYI both are separate skins, and separate planes. In AH2 go to hangar, click a20 and save it, then go look in a different folder. It will be found in there.
*IF* they are the same layout, I say try porting it to the a20. If not, then you will have some work to convert the boston to the havoc.