Superior Air Parts had a proposal to market two versions of a three cylinder opposed piston aircraft uncerified engine as recently as 2015 rated at 100 and 125HP, seems now to have been withdrawn as their web site now makes no reference. As for unusual or unique, no, I wouldn't call sleeve or desmodromicvalves, or turbo compounds unique or unusual either. Perhaps I've lived too long and seen these "unique and unusual" pieces of machinery in daily use earning some body a living.