Mossie was never locked.
That said, if it had been there are other low ENY things I was comfortable flying. Locking yourself into an ENY 5 fighter seems willfully self limiting as it is known that those suffer frequent ENY locks. If you're going to master a single plane, at least pick something ENY 20ish.
As to the plane actually being discussed here, the P-51D, I've never understood why the P-51B is not an acceptable substitute. It is extremely similar to the P-51D. Closer than any two A6Ms, Bf109s, Fw190s, Spitfires or Yaks are to each other.