The YS 120 AC would be the best choice if you're determined on running a 1.2 ci. four stroke in the Ulti. This plane weighs in at 12-13 lbs and there is a lot of drag being a bipe. A 1.2 will fly it, but not with authority. A YS 140 would be a better choice, but the 160 DZ would be the ticket.
If you wish to use a two stroke I would go with an OS 160 FX or a 1.8 Moki.

I have a Saito 180 in my Ultimate. I believe it weighed in at 12 Lbs-12 oz. It does fly quite well with very good vertical performance, but slow to pull out of a hover with a APC 18 x 6. If I had to do it over again I would go with 160 DZ.