I have been asked a few things like this over the years, I always describe them as "Grey area's"
Depends how anal people want to be on the day.
One thing about doing this with the CZ's though is that the frames are different.
The early frame has the front downtube bend and go underneath the motor where the side cradle tubes bend and join into it.
The later frame has the front down tube straight and finishes just before the bottom of the engine whereby the side cradle tubes go further forward and bend upwards at an angle to meet the donwtube.
What this means is the downpipes on the later frames can fit nicely between the side cradle tubes and so it can tuck up nicely.
On the early frames however that front down tube that bends underneath the engine gets in the way forcing the pipe lower to the ground.
It can be done, I made a pipe like this about 11 years ago but the front section sits about 40 -50mm lower on the sidepipe frames.
I have blown up some of VMX60 photo's to help explain my words.
Early Sidepipe frame
Later 72? onwards frame