Piper PA-48 Enforcer - originally to be P-51 with a Lycoming T55-L-9 turboprop engine.
By the time the PA-48s were completed, they shared less than 10 percent of their structure with the P-51, and were longer and larger.
The first two prototypes (N201PE & N202PE) were heavily-modified P-51s, N481PE and N482PE were new-build airframes.