Putting the old bus body on a new school bus chassis might be an option. Finding one that's exceptable (gas or diesel, auto or manual, air or hydraulic brakes) shouldn't be to hard (I think most school buses are retired after 10 years of service).
Mike B
Yes, you can find a good used DIESEL, AUTOMATIC, with AIR BRAKES school bus from $1500 to $3000 used on eBay. That is a STEAL for what you're getting (chassis and drivetrain). Unbolt the old body and bolt it on the newer running gear. I'm sure modifications will have to be made, but I don't see why it wouldn't be that difficult to pull off.
