I've had a similar sound on another older car before the rod shattered and went through the cylinder block. Was rod knock from a bent rod.

Rod knock in this case is one possibility. Could also be a bent valve and/or broken spring.

Isolate the source of the sound, via stethoscope/screwdriver/etc. If its engine related do a dry and wet compression test. That will tell you if its head or block related. Rebuilding a head is a lot less work than swapping out an engine.

T.