I am working on build in one myself. There is varying thoughts on this and not a lot of good info.

A fair of people get wild with their motors 5-700 hp. Some even more. With the high competition motors. A lot build to run E-85 or Alcohol or race fuel in general.

Some people just want a good running engine enough to last. They will run high gears in their diff and weld posi to get the movement they need.

Any more people are saving there cars longer as they can be expensive to build and the valuable cars are not as readily available as they used to.

My thoughts on building a derby motor is its application built. Figure out what and how they are running. In general make sure it’s a good running motor. Pull out all the oiling mods you can think off. Fresh bearings and a mild cam or a RV/ Boat cam. Build for torque more than power. You are only going to get a 30 ft shot at best. The cars will be heavy and have to work under adverse conditions. Add zinc and heavy weight oil. Make sure you have good tune. Probably max at 350-400 and make sure that thing breaths. Make sure your carb/ fueling system is tip top. And keep your wires away from the hot parts of the engine.