The XUV 550 has a 570cc Briggs & Stratton Vanguard engine.
Here is the Briggs website for it. Surprisingly I think the engine is made in Japan by Mitsubishi.
The next model up, the 625, advertises a 23hp liquid cooled Kawasaki though I have never seen nor heard of one actually existing. There are tons of 825's, some 855 diesels and 550's (what I have) but I have never seen or known of a 625 outside the brochure. I think the 625 was previously the base model, very similar to the 825 but with a smaller engine. In 2012 the XUV's got a makeover and the 550 became the new base model. I assume they keep the 625 on the webpage because there are still some unsold units floating around. The difference between the two styles is really noticeable in the ROPS. The front bar on earlier XUV's comes down in the middle of the hood, meaning you can't open it with a windshield in the way. The newer 550 and RSX have the front bar come down behind the hood. I think primarily so a windshield can be fitted without blocking access to the hood.
What trouble did you have with the engine on your Cub Cadet?