I haven't flown the Harrier but I have a lot of experience with the UCD. It hovers very easy. I don't see how the Harrier could be much easier. I think the YS63 would be a good engine but I think you'd be disappointed in the OS70. If not the YS I'd go with the Saito82. That's the engine I've used and I like it. Some have even used the Saito 100 or Saito/Magnum/OS91. All of those engines seem to work good. These UCDs and I'm sure the Harrier also are really fun planes to fly. You won't be disappointed.
Thanks
Barry