Of course you can, and they did it in Boulder. From the Boulder News,
What’s 30-feet long, bright orange and runs on electricity? Boulder’s newest old bus, the first in its fleet to go fossil fuel-free, courtesy of a Front Range company specializing in conversions that are both cheaper and faster than buying brand new.
Via Mobility Services, which operates the HOP line for the city of Boulder, commissioned and paid for Bus No. 15 to be stripped of its non-functional diesel engine and outfitted with an electric power train. Lightning Systems of Loveland performed the retrofit; Longmont’s UQM Technologies provided the electric motor The process cost $260,000 and took roughly four months.
The bus plugs in overnight; one charge powers a full day’s route.
Everybody needs to do this now.