FDR or CVR
Unless an aircraft is already wired up with the required sensors and possible instruments it would be quite a task to install a FDR but not impossible. However, a CVR or a Video Camera trained on the Instrument panel would suffice for an interim period. Back in the day, and I have been retired too long to know about modern technology, the CVR's on BHL aircraft had four channels one of which read the Rotor RPM, which might have been useful in the pub accident investigation.