Andre, with something that old, you would have to disassemble and rebuild. You'd probably find that some parts were corroded simply by the passage of time in the presence of ordinary levels of moisture.
How would a 30-year-old engine look inside if it hadn't been run in all that time? It might be useless as a result of rust. Plastic/rubber seals will likely be hardened and cracked or crumbling. The same might be true of the rotor head and prop. Old wood props will occasionally fly apart as a result of rot that is not obvious on the surface.
Bensen metal blades are inefficient compared to more recent blade designs.
The value of a VW engine is greatly dependent on the quality of the conversion. If it's just a junkyard car engine to which someone added a prop flange on the fan end of the crankshaft, it's worth a couple hundred dollars at most. If OTOH it's a purpose-built aircraft engine such as a Revmaster (involving a bearing-supported crankshaft extension, magneto ingintion, boring and stroking, different cam, oil cooler, special case/heads/carburetor, internal balancing and magnafluxing etc.) then it's worth some money if it is still in good condition.
Without investigating these issues, I'd be afraid to pay more than $1000 US. If, by some miracle of preservation, the machine proved to be in flyable condition after careful investigation, it might be worth $5000.