The Catalina 30 is a very popular boat and has a big following.  We have repowered a lot of these over the years and the Beta 20 is the right fit.  The Catalina 30 only weighs around 10,000 lbs so it doesn't need much to push the boat to hull speed.  The other thing to consider is prop size.  Most Catalina 30s cannot fit a prop bigger than 13" in diameter.  To use more horsepower you will need at least a 14" diameter prop.  Also if you look at the specifications of the Universal M25 it's continuous rating is 21 hp.  The Beta 20 has a broad torque band that actually puts out a bit more torque than the old M25.  I hope my explanation helps and if you need more information please feel free to give us a call.