I test flew the (first and only) HKS700E engined X'Air. Wonderful was not a term I would have applied - it was basically a 4-stroke Rotax 503 - massively underpowered for the class of aircraft, with complications and extra weight that in my book made it a far less good weight than the 503. To be fair, it was very frugal on fuel, but I can't recall any other redeeming features.
IIRC, the first production HKS engined AX2000 kept blowing up CDI units, until they realised that a common earth through anodised airframe components wasn't conductivey enough for the job and just separately earth strapped everything together.
G