There are many videos on YouTube about these machines with excellent information on all aspects of improving them, including adding a motor.
The best solution when adding a motor ensures that you retain the hand cranking ability.
A big advantage of motor drive is being able to use both hands at the workpiece.
A useful motor will be in the 500-750 watt range, with a controller and a needle positioner.
The brushless types on eBay are currently £75-£120, depending upon size. 1 horsepower = 750 watts.
3d printed conversion kits are available for the motor conversion, about £45.
It's all on YouTube, a guy called Henskecht does the best I've seen, he prints the kits.