Engines are so expensive because many were tossed in the garbage back in the day. After Simplex closed down, no parts were available for 25 years! It took several people in the 1990's to get some parts made. It seems that all those people are either dead, or out of the simplex parts business, with the exception of Wayne. If anyone knows of others producing parts for sale, please post the info!
That being said, some parts are easier to find than others. The are no new cases, cylinders, or flywheels/magnetos being made. Just too cost prohibitive. Wayne seems to have most other parts available, including billet rods, pistons, flywheel covers and he even has a new crank available.
I know the person who makes the handlebar grips passed away several years ago. The widow is still making them, but is old, and wants to sell the business. Unfortunately, I understand she wants quite a sum for the business, and has found no takers.
Gary Wollard estimated that there was twice as many bikes out there as engines! This is one of the reasons Wayne just developed a kit to put a honda 6.5hp engine with torque convertor in a Servi-Cycle.