Any quality 50cc engine has more than enough power for the Ultimate. The larger and heavier engines , although having more available power than a good 50, present balance issues with the Ultimate. At least that's my experience and opinion.

I used a Brison 3.2 in mine and had to work a little to get the c/g far enough aft for proper performance levels. I was surprised in reading the posts of others that a similar condition was experienced with the DA 50, which is a couple of ounces lighter than the Brison. That's the reason I would suggest a 50 over something larger and heavier. You really do not need any more power for this plane than a Brison, DA, BME, or Taurus 50-52cc engine provides.

Pat