So I went to my dealer to ask if he could make a test fit for me but no, changing the sprocket is too involving and they won't do it on their demo bike. He told me that it may work and he will ask confirmation from Zero but even if they say no I would not fully trust the answer.
Dealer also told me rear wheel is the same, even if tires are different. After checking this is not true as the DSR/X wheel is 4.5" wide and the SR/S wheel is 5.5".
But I took some measurements :
SR/S
Center distance: 520 mm
Distance between two rear sprocket bolts: 143 mm
Center distance between two rear sprocket bolts: 158 mm
False diameter (not by circle center but aligned with two bolts): 310 mm
DSR/X
Center distance: 515 mm
Distance between two rear sprocket bolts: 14.1
Center distance between two rear sprocket bolts: 158 mm
False diameter: 315 mm
So I am 99% convinced that the front and rear pulleys can be mounted. But there's still the question of alignement.
Looking at part catalog, there is no reference for the swingarm for the DSR/X, this may indicate that it is the same as for SR/S and SR/F and if it is the same, the alignment has to be the same regarding how the motor is mounted and the rear pulley have to fit.
Regarding the clearance between swingarm and pulley it is about 4.5mm, which is not enough to offset the pulley by 5 mm but the flange on the SR/S is 4.5 mm thick, it looks like it is thinner on the DSR/X. If they manage to reduce it to 2 mm, it would leave 2 mm of clearance with the swingarm, that would be the bare minimum !
I'll try do some more measurements on the DSR/X next week regarding the swingarm clearance, if the theory is confirmed, I will need to find someone who is willing to let me test his DSR/X rear pulley on my bike. I think swapping only rear pulley (keeping stock front pulley and belt of SR/S) will be enough to know if the conversion is possible or not.