anatak wrote: I tried to loosen the oil plug with a 21 metric spanner <snip>
Don't. Use a socket and try to 'shock' the plug loose, i.e.
don't apply gradual pressure. The metal of the bolt head is soft and will readily round off if you try to loosen it slowly - especially if you use a spanner.
anatak wrote: <snip> ... with the exhaust being in the way
Take it off, if necessary - sorry!
anatak wrote:Is 21 metric the correct size or is it some arcane imperial size that I need ?

. It's metric - can't remember what size. The only imperial fastening on a Ducati is the pillion foot peg nut, for reasons never really understood ... (oh, and all Brembo components which are also imperial altho' always translated to metric).
anatak wrote: ... does anybody know the correct torque values for tightening the plug afterward ?
This is kinda the wrong approach. This fastening isn't stressed or 'subject to vibration', so a torque value isn't really what you're after. And, as mentioned above, the bolt head is soft, as is the material into which it is threaded. So either one or other is easily stripped by over-tightening

.... This, you do not want

.
What
does matter is the condition of the sealing washer. If the one you have is copper, anneal it and re-fit. If it's aluminium, replace it with new. (You may get away with re-using it, but it's your call. For me, it's a 'No Brainer'). Then, just nip the bolt up tight - and not super tight, neither!
anatak wrote: Or even better a manual with the common torque values for the beveldrive 900 square case engine ?
Ducati were not great at listing out torque values - they are there, but you often have to work thru the pages of the workshop manual to find them. But, that said, here are some basic values from the Dramah manual:
6mm: 5.8 - 7.2 lb/ft (0,8 - 1,0 Kgm)
8mm: 10.9 - 14.5 lb/ft (1,5 - 2,0 Kgm)
10mm: 14.5 - 18.1 lb/ft (2,0 - 2,5 Kgm)
As you'll see, they are pretty much the standard values for 8.8 grade steel metric bolts.
HTH
Craig