The Xy 26 engines are made in 3 versions (as listed on the website today). All three should work but either of the 2 with the slanted spark plug I think might be a better fit in the cowl. The "A" version with the straight plug might require clearance holes. They are reported to as strong as the Zenoah or maybe a bit stronger and they are fairly inexpensive.

But if you want a bit more reserve power or to fly more non-scale maneuvers, go with what GageeBees suggests and go a little bigger, the 50/52 may be a bit much.