Would a ST2 (raw water cooled) Lister be of any value as a "parts engine"?
Not unless you have another STW2 engine, as most of the water cooled components are specific to the STW range (some items will fit the Lister SRW engine range from which it was originally developed), and the expensive/hard to get bits like water cooled exhaust manifold and the top water rail are specific to the STW2 engine (these are the bits that usually rust from the inside out on raw water cooled engines, that have not benefited from having had frequent sacrificial anode replacement). Further more due to its low profile flat top design the injectors are unique to that engine, although the injector body is the same as the Lister TS range.
Here is an example of the top water rail on ebay uk at present:
http://www.ebay.co.uk/itm/New-Lister-Petter-202-91280-Water-Rail-Manifold-STW2-STWM2-202-91040-Marine-/131854010389 and if I had a STW2 or SRW2 engine. I would buy both for spares as that is a good price.
However back to your inquiry, it is highly likely this STW2 will be fitted with the Lister LH150 hydraulic gearbox, if for nothing else get it for that item alone, as good s/h LH150 boxes are now becoming very hard to find, and in the UK reconditioned versions go for telephone number prices even with a core exchange.
The STW like its SRW relative was originally an air cooled engine that Lister's converted to water cooling [much like the VW Wasserboxer DG 1.9 flat 4 engine was a converted air cooled engine], they can and do leak prolifically on the head gaskets (I use the term head gasket loosely because it is a combination of steel shims and rubber O rings). If not attended to a leaking head gasket will then contribute to the cylinder head / block studs seizing in the cylinder blocks, making cylinder block removal an interesting process.