Robin,
I recently bought some parts for an SP3 from Ducati. For things like the piston rings, head gaskets and shell bearings the system showed the old part numbers superceded by more recent 916 parts and all in stock.
The SP5 barrel is the same as an SP3. I got a pair of used SP5 barrels and pistons for UKP 70 on e-bay a few months ago. Worth keeping an eye out for parts there.
Mike
851 SP3
996 SPS
Brutale
Threaded Mode