The Kawasaki 440 series engines have been used successfully on ultralights. I've never heard of a 398 but I suspect it might be too small for a gyro as they're pretty inefficient. Most gyros with happy owners seem to have 60+ HP.
Have you any idea how successful censorship is on TV? Don't know the answer? Hm. Successful. Isn't it?
Yes the Kawi most often used in the Gyro's would be the 2cyl, two stroke air cooled Sled engine. The only actual test I have seen is ONE Gyrobee ( a true 103 legal one) that ettempted to use one, he got it to fly but ended up putting a 447 Rotax on it in the end I believe. The issue is not the Horsepower per se, it is thrust. the gyrobee series for example tended to not perform acceptably with less than 40+ pounds of static thrust. In this case I think the redrive and prop matching finally became to much of a hassle in the end.