Hi, and the UCD .46 is a very pretty plane and should be a good addition to your fleet.
I know little about the UCD .60 although I have one still in the box. My friend, Dave Quatse at the Silver Spring Field has an O.S. .91 FX in his UCD .46 and loves it. I have seen him tail touch his plane and hover with it. I read a review wherein the O.S. .50 FX was tested and although it did well, the reviewer liked the O.S. .70 4 cycle. He thought the O.S. .91 was too throttle sensitive in the range he wanted to operate.
I bought a Saito 100 for my .60 UCD mounted it on a board and the engine wouldn't run properly, so much vibration the carb broke off the back plate. (Yeah, my APC prop was balanced.) I sent it back to Horizon and they sent me another motor by return post. Yesterday, I mounted it in my .46 UCD and ran 2 quarts through it. This one also vibrates like hell and doesn't want to run smoothly. Man does it have power. Way too much for the .46 UCD. I haven't called Horizon yet, but the valve gaps were and are perfect. The only thing else I can think of is that the Saitos I got were assembled improperly re the valve cam. I have two other Saitos a .72 and a 1.5 and they are only O.K. I've been in this hobby for over 20 years and have never had motors act like these Saito 100's. I have purchased my last Saito.
I am having a lot of fun with my Twist.