I managed to contact via email BeTon, he kindly gave me some more infos about this two motors, and than I replied to him:
After a better study of your photos, I solved the problem:
You owned an almost original dr 600 engine, with a few swaps:
the barrel was from a 90-95 dr 650 or djebel (same engine, SP41, 42, 43, 44, 45 are almost identical) wich has very similar bore (95 mm vs 94,5 of the 600, in effect, many 600 owners use the std bore 650 piston instead of the 600 +0.5 mm because it cost less, has graphite on top, higer compression and better oil feed than the 600's one), but 5 mm longer stroke.
It's easy to understand why your barrel was machined, because of the longer stroke = poor compression if not milled.
To tell the difference between first version of 650 barrel and the 2nd generation, is sufficient to look on the left side of the barrel, the fins are machined to let pass a big pipe from the head to the block, while in the newer models that pipe just doesen't exist.
So to make a long story short: SP 46 sadly can't be fitted on 600 block.
Thanks a lot anyway for your time and for your passion, and for the fast reply