VIN # question

[74 C-30]

my old truck was originally bought by my grandfather in 74 (still titled to him although he's dead) and was been registered in my fathers name from the late 80s until 97 and then again in 05. when my dad swapped cabs he thought he included the vin plate but theres nothing on the door jam so im assuming its long gone. is there anyway that i can legally solve this and try to keep the original VIN? all title, registrations, spid should match up, now can the engine or frame be used to identify too? thanks for any help as if its not worth it to fix it then i wont