The advantage of Saito 120 for UCD over other engine is that it's rear exhaust, and can utilize the exhaust channel below to keep the tail servos clean.

Also, Saito 120 is the lightest 120 you can find. With 16x6 APC prop, you will have a perfect combo.

Make sure you build a battery compartment at the tail to balance out the 120 up front. With a regular 91, the balance is about right with battery above trailing edge. The 120 is about 10 oz heavier, and a 3 oz battery pack at the tail may just balance out the 120.