I flew my kit version for a few years on a Super Tigre S90 with Zinger 14x6 prop. Not quite unlimitted verticle, but good climb rate for 400-500 feet. I saw one fly with a modified YS120, bored out to a 140 the guy said. It didn't really fly, it more or less was drug around by that monster! He'd go verticle forever, and I watched his throttle stick stay at about 1/2 point the whole time. Quiet as you can imagine too, since he was truely only running between 5k and 6k rpm. It was swinging an APC 16x8 prop, don't know what kind of fuel he ran.

My vote would go to a YS120. I'd planned on switching mine out, but decided to trade the plane for something else.