Interesting looking bike! Looks like there has been some good work done on it in the past, especially the air filter arrangement....I assume theres some sort of central oil tank behind it taking up the space where the battery would normally be? That would be a problem if you wanted to fit lights....also the dynamo being absent!
If it were mine I'd certainly keep it much as it is, changing it back to a standard SS would involve finding lots of parts...
The tank is from a later unit single BSA but unsure exactly what the brakes are....maybe the same?
Its possible that when it was first built (considering the design of it) that an unusual gearbox might have been used.....see if you can see any letters stamped on the top of it.....SCT for instance!
As to value......what anyone is willing to pay really....speaking for myself if I saw it on ebay at say £3000 I'd think that about right!