Graham is right, you can not build a new engine for £10K
The cases, heads, muff/liners, pistons, head parts, rockers, flywheels, gearbox, covers come to about £10.300 and that is only the big bits.
Add on about £5k for the little bits, A BTH @£550, an Alton @£300, carbs @£600 and you are looking at £17.000.00
Pete, did you work out what your engine cost to build?
I'd call it indecent rather than decent money. The buyer may spend a minimum of fifteen or twenty grand restoring it and then we're getting up there into the realms of sixty grand for a Black Shadow. At that point, it really does start becoming viable to consider building one from modern parts, given than it would cost less than thirty grand, as the VOCSC experiment showed us.