Actually it is and you can.
When you are charging/barter someone to convert an engine for them, its important to consider the cost going in and exiting.More often than not you will recoup more of what you spent by using an off the shelf solution than a conversion when selling the plane or engine.
Some guys want to use a particular engine for the sake of using that engine, they have no choice but to convert it, but for someone just looking to power a plane. They can grab a Zenoah cheap, bolt it on themselves and it will retain most of its value after a season.You can spend more on a conversion and wait forever to find the right guy to buy it for 1/5 of what you have into it.