Okay, show us a trainer KIT, AND NOT AN ARF, that is designed for electric power.
Years ago, I bought a Goldberg Electra motor glider kit. When I inquired about what motor to use, instead of the big brushed 550 can motor that it came with, I was told to buy a motor that equaled 100 watts per pound of weight. Since a nitro powered Kadet Sr is a 6 lb plane, a 650-700 watt motor would work. If he doesn't want to go that powerful, a Kadet Seniorita is only 3.5 lbs so a 400 watt motor would work. Want to go right into the middle, the Kadet MKII is right at 5 lbs so a 500-550 watt motor would work. One of the recommendations I got was to use the following, based on an all up weight of 48 ounces, the listed weight of an Electra:
Rimfire .15
35 amp ESC with BEC
a 10X5 folding prop
a 3S 2,800 lipo pack
Those recommendations would work fine for a Seniorita. Throw in an 11X6 prop to match the size of an equivalent nitro motor and you would be good to go