Frankly, I think Kelly Blue Book is waaay off and doesn't really know what they're talking about. Prices vary from one locale to another. That said, I really don't know what those monolevers sell for - I rarely see them, so if you want one, you pretty much have to pay what the owner is willing to take. As I recall, a nice one here was advertised at around $3K. Maybe $3500.
A frozen engine isn't caused by old oil!

Gimme a break!!
Normally it's caused by condensation in a cylinder rusting the rings to the bore. Since everything is pretty well closed up on these bikes, the only realistic guess is that moisture got in there through an open exhaust valve. I'd think there's a pretty good chance it could be freed, cleaned up and run without much of a downside.
But you are taking a chance. Good thing is that R65 cylinders and heads tend to go pretty cheap on ebay.
Probably the most scary prospect in this scenario is the transmission. Oddly enough, the engine doesn't bother me, it's the transmission! A much more expensive proposition if it's got problems.
Some specifics would help - and we'd be interested in knowing the asking price. Oh, and welcome to the forum!
Where are you located?