It may be. It all count on details. If you could take stock air cooled head, mill it and attach to custom made cover - it may work. But it will add few kg more.It has been mentioned a few times, I brought it up in one of my first posts on HBA. I'll just say there was no applause. In theory it would be a good way to move heat from the exhaust side of the head to "somewhere else" (?) where it can be disposed of, but the practical considerations of doing that are significant.
But - on this side of globe - it is way cheaper to buy used rotax912, check it, make all dimensions etc. And you know what you have. I have similar dilemma for myself - but if you do not have vw motor / bmw boxer or suzukis on hands - you are spending at least 1k on bare used engine, which you will have to tear apart and rebuilt. + gear box / prop adapter. And no-one will give you any warranty.