i'm trying to find threads, but as i always suggest to budding mechanics & aspiring gearheads, as well as rubbertrampers in general with no automotive experience...
*especially* those starting out on VWs,get a hold of
"How to Keep Your Volkswagen Alive: A Manual of Step-by-Step Procedures for the Compleat Idiot".. this book is great...
i dunno any particulars about the Vanogon, but i had a couple of buses.. a 70 & 78 (???) maybe 30 years ago (in my 20's).. and what i learned from this book, rebuilding my '61 bug back in 1978 (in my teens) is *still* applicable..
John Muir (an engineer) wrote the damn thing, and it's a gas..
the 1st edition of the book was published in '69 & was updated through 2001 or so..
https://en.wikipedia.org/wiki/John_Muir_(engineer)
good, good stuff..
the book is geared mostly towards air cooled engines, but if your rig is older than 2001, i bet there are applicable sections.. i *think* the later eds had some stuff about the liquid cooled systems, but i don't know that as a fact..
the book is geared mostly towards air cooled engines, as @Eng JR Lupo RV 323 said "Old VW engines are a great place to start, if you're not very mechanically inclined
yet. ...So take a few hours or days even, it's like a glorified lawnmower pretty much."
in the earlier editions, Muir made a specific reference to loose head bolts on the classic motors, more or less being: "they can make it sound like Thor's Hammer is banging around in there, but all you need to do is torque them to specification".
if the bolt isn't *in* the cylinder (???), or if it wasn't run too much since it fell out, you may just need to replace it, if it's missing.
There's a chance the bolthead could have sheared & gotten swallowed, but if the head is not so loose that you have more than 3mm or 1/8" of movement, i would just bolt her down & give her a try, if you're going to drive her locally, for the most part, before going on any long trips, just to see how she behaves..
they are easy to drop & work on, if you have a small floor jack & jackstands, so even if she purrs sweet, go over her before heading out..
if the damage is done, it's done, and you'll also learn from seeing what it is, and how it happened, as well as repairing it..
good luck!!