You are right. This certainly is rehash, but humor you anyway
From my experience the most popular engines for the UCD 60 are
[list=1][*]Saito 100[*]YS 90[*]OS Surpass 90[*]OS FX 90[*]Saito 90[/list=1]
Those 5 in my opinion are probably the most common. With that being said I've also seen people using Saito 120-180, YS 120, 2 stroke 60's, OS 1.08, and even RCS engines.
Now that the YS 110 is out I'd suspect you'll see a ton of people going with that engine. Give it a little more time and it will pass the Saito 100 as the most popular engine for the UCD.