I found some brand new looking cng buses when I was bus shopping. After asking around the consensus was that with the limited fuel sources, expiring tank certifications, and some other factors I've surely long since forgotten, that they just aren't viable for most folks looking to convert.
If leaving town in the bus isn't something you care about there are some tremendous deals out there now and then.
I know nothing about the Deere motor other than it exists and I've seen it in bus listings. But when I worked at the fab shop we used several different engines in our trucks and the John Deeres were expensive to buy and to fix.
and a CNG compressor for home use starts at about $5000 and compresses 1 gallon equivalent of gas in CNG an hour, still kick myself for not buying the crown victoria from the city hear for $1500 that included the compressor