The best bet for the 46 UCANDO is the OS 70 4 Stroke. Hands down.

As in best bet, I mean well rounded. The Saito 91 is overpowered. Not that overpower is not good to have, for this plane you just don't need to have it. The UCANDO's are made to do very slow, graceful aerobatics, the 70 will hold hover and pull straight out. The 91 will do the same, but if you ever get the urge to ring it out full throttle level flight, you will flutter your wings and rip em off with the Saito 91.