It may be the original engine, but still be reasonable about your intentions and options.
Perhaps it cost $3-6,000 to professionally rebuild. Or you may find a good replacement for well under $1,000.
I enjoy seeing all original trucks and restorations, I’m glad some people are that passionate and dedicated. There is a lot to learn from their research and efforts.
I’d also much rather see a good everyday driven truck with slight and or sensible upgrades than a half hearted “original restoration” that actually isn’t original.
Go all the way and be original or be practical (and drive it a whole lot), I don’t see much use for half way in between. I really enjoy driving old trucks, and 100% original isn’t necessary for that. Not saying you can’t drive an original as it is or fully restored to like original because they were new once and perfectly driveable then as now; but consider cost and rewards.
Decide what makes you happy and fix or restore your truck to suit. Also step back and reevaluate that plan a time or two along the way.