It makes no sense to convert a (still relatively new) CNG bus.
5075 was just a transmission swap.
7746, 7691, and 3563 were both engine and transmission swaps.
Both are relatively easy to do, compared to an entire conversion. The CNG tanks, fuel lines, engine, transmission, etc. all have to be removed and replaced with the proper battery stuff. It's a lot more complicated.
Any new bus coming from New Flyer is automatically an Xcelsior, that's all they offer
...not counting the MiDis lol