It's worth what your prepared to pay for it
However if you decide to buy it, make sure you thoroughly inspect it before hand, as the genuine spares situation for this engine and its single cylinder CD counterpart is now getting dire, unless you have deep pockets.
Having said that their is some parts interchangeability with other Lister engines, ie piston rings, fuel injectors, and valve springs.
I also have some NOS genuine spares ie piston, head-gaskets, and valve guides. I can also source big end bearing shells. PM me if any of this sounds interesting.