I use a Sullivan Hornet starter for the Cox .049 engines, but that is too big and not convenient for starting Cox .010 and .020 engines. So I made the starter shown in the attached photo (shown with Tee Dee .010 engine). Works great. I routed into the wood for mounting the motor and battery to provide a better profile to hold. The red/black plastic part is a high current microswitch, with its button underneath for easy activation when holding the starter. The cup to mate with the engine spinner is 1/2" diameter aluminum rod that was drilled for the rubber cup (thick wall surgical tubing) at one end, and for the motor shaft at the other end, and tapped from the side for a set screw to hold the motor shaft. The motor is from Amazon www.amazon.com/gp/product/B01M58POHF/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1
The battery is a 3S LiPo.