1991 Mitsubishi Pajero noisy engine

The actual tappets could be worn themselves,
only way to be sure is strip them out and eyeball the ends.
Or it could be diesel knock,
Sounds very like tappets,
Check the pump timing.

With an engine as old as that you should always hear it running and checked for wear / noises before buying and fitting .Every possibilty knackered engine after 25 yrs .

How do I check timing and what is diesal knock ?

Diesel knock is basically the injector triggering at the wrong time in the cycle i.e not injected at the correct point BTDC.
Too soon or too late.
So the injection pump cam timing has to be checked.
usually by pinning or lining up the crank/pump marks.