Might be, but fixed gears sometimes can be problematic at mounting the engine
A few weeks ago, I decided to build a motorized bike for my friend. I initially planned on restoring an older mountain bike, but later found out there was too much work involved in that bike, and I'd rather sell it for parts.
I ended up finding a good deal on a vintage schwinn cruiser bike for $20 (Had lots of surface rust, but most came off pretty easily). All I really needed to get the bike itself going without the engine was a front tube.
I expected the most difficult part to be mounting the sprocket on the rear tire around the hub, which is usually what I find difficult to get true. The coaster brake also makes it harder to mount the sprocket, it was mounted backwards so the brake arm can clear the chain properly. So I took it to a backyard motorized bike dealer I knew, and he charged me $25 to do it. The hub feels alittle loose though, but it rides fine.
The most annoying problem I faced was trying to mount the engine. The stock front sprocket was pretty large, and wouldnt clear the engine. I ended up getting a bmx sprocket(Should have thought of it earlier and saved myself the headache), and shortened the chain a little(Which I need to do again and take off the masterlink lol), and it made it easier on me.
Point being if u ignore the long story, is that it might be possible, but you'd likely have to get another crank, and the coaster brakes would be alot less effective if u tried to drive the chain side, and brake at the same time.