Complex sentence: The main difference between the Walrus and the Sea Otter was in the mounting of the powerplant; the Walrus had a rear-facing engine with a pusher propeller and the Sea Otter's engine faced forward with a tractor propeller.
Split into simpler sentences: The main difference between the Walrus and the Sea Otter was in the mounting of the powerplant. The Sea Otter's engine faced forward with a tractor propeller.