If you can find one, a 16 HP Champion, or, better yet, the class B "Hotrod" racing version of it. Both these motors are getting hard to find, but Champions had a reputation over the years of beating out Mercurys, and they are light-weight.
After Champion went out of business in 1958, employee Lyle Swanson bought all the parts and continued to make the racing version until he sold rights and parts for itto someone else around 1980. These were known as "Swanson Hotrods". His repair shop and stock of Champion parts was just 15 miles from me. After he sold rights to the hotrod, He told me he had a lot of fun building, selling and supporting racing of these motors, but he "got tired of arguing with Mercury’s lawyers every time on of his Hotrods beat a Mercury". This looked bad for Mercury that a small, independent shop with an old 1950s desing was still beating new Mercury racing motors into the 1980s.
Dave