Supereri,
The choice of the Saito 100 4-stroke engine for the UCD 60 seems to be a popular one. I'm currently removing an OS 91 2-stroke from a UCD 60 and installing a Saito 100 4-stroke with a 15x6 APC prop and I cannot at the moment confirm that this will be a better setup. I hope it will be.

I'm using only Futaba S3004 standard servos on all three of my UCD's (two 60's and one 46). I thought I'd give them a try before buying fifteen $100 servos for the control surfaces and so far I see no reason to replace them. And yes, those S3004's with their puny little 44 ounces-inches of torque will crank that big rudder over and do knife edge loops, blenders, etc.