You can convert a DC motor to run on AC power by using a bridge rectifier rated at 5 to 7 amps and at least 50 volts. Must be a bridge rectifier so both sides of the power get changed to usable DC voltage.
AMCDave; your total investment may be closer to $20.00 then just $2.00 for the parts at today's prices.
The new engines by MTH have electronic circuits in them that read what type of power is being used: whether DCS or DCC or conventional AC track power.
With an AC powered engine I would not recommend using DC power because the horn or whistle may sound off continually.
Lee Fritz