This is going to be an extensive conversion. It is not just a matter of converting the stove; the entire system needs modification. First, you will need to store the propane
in a vapor proof locker. The locker itself runs $600+.
Unlike CNG (which is lighter than air), propane will accumulate in low parts
of your hull
and will present an explosive danger
. You also need to replace the CNG regulator
with a propane regulator
Here is a link to the AYBC regulations
regarding LP installations on boats.
If your boat has a CNG system, it is likely going to require extensive modification in order to be compliant.