For the above, take the plane you want to convert to electric power, remove the engine, fuel tank, and battery pack.

Weigh the plane as it is, without the engine, tank, and battery pack. Include the receiver and servos you plan on using. If you are using the same ones, then weigh the plane with them installed.

On the internet, find a candidate motor/esc/battery and look up the weight of this combination and add it to the weight of the plane.

Do the math for the above post and figure out of the motor/esc/battery you found on the internet is a workable candidate for the desired type of flying you wish to do with that plane. If it all works out, then get it and install it. If not, find a different motor/esc/battery combination and work it out again until you find the right combination. This may take you all of about a half hour to do.

CGr.