Horsepower (which has many definitions) is the torque output multiplied by the engine RPM (divided by some number to make it equal to the strength of an average horse). Since these engines operate in a relatively low RPM range, the HP number is not that high. Most countries switched to using kilowatts as a measure of vehicle engine power.

Also remember that these are diesel engines from the late 1930's. This was before the development of 2-stroke (blown) or even turbocharged diesel engines. These engines were a lot less powerful compared to modern diesel engines.

The engine actually only made 150hp, but luckily there were 2 of them.

Yes, they were certainly less efficient than modern engines. However, ICE engines have always had trade-offs between power output per size, mechanical efficiency and reliability, and for this vehicle the last one was certainly of most importance.

Horsepower are only one of the measurements for an engine. These engines have enormous amount of torque which is more important than sheer horsepower.

Diesels like this trend to produce relatively low horsepower, but lots of torque.

As a comparison, here's a modern 11L diesel:

https://www.macktrucks.com/powertrain-and-suspensions/engine...

You'll notice that HP is 325-425, so not that dramatically different from the Snow Cruiser.

They lacked turbo charging which is necessary for diesel to make modern levels of power due to the low max rpm of the engines.

Naturally aspirated diesel where relatively efficient they simply lack power.