Yes, a lot of inexpensive ebikes have 12G spokes. Hard to maintain tension. They sell double butted spokes which are 13G at the elbow and 14G at the nipple. They will give good tension. If the motor holes are too big for 13G, washers are available.
The nearest bike shops always seemed ebike unfriendly to me, so I didn't even inquire about wheel building. I just jumped in and learned how. It's not that hard if one is methodical. I just used a spare fat bike fork (135mm axle width) clamped to my desk as a stand. I watched a youtube video where a young lady aptly demonstrated the single cross pattern used for ebikes and had another ebike wheel to stare at for a model, when I got lost.
My biggest problem was getting the spoke lengths correct. I used the Grin spoke calculator on the ebikes.ca website.
If only one wheel (I've done six now) then having a shop do it is a good idea if you can get someone who is not afraid of a hub motor.,