Sound like a terrible situation to be in. We understand your concern. Unfortunately however at this point unless the seller is willing to buy this bus back from you your only option is to bring the engine emissions system up to compliance for as cheap as possible.
This would mean determining the engine make and model the bus is equipped with and purchase & install the emission components which the California version of that engine has... assuming the California version for that engine exists. You'll need to contact the maker.
posted by SmogTips Support