Re: 1951 Schwinn Panther - maybe?
Nice! Yes, that is the staright bar frame which most of us call the Panther. Actually this frame was used for decades and the panther model is probably the most well known as it was a real beauty. But there were others as well including the "motorbike". The differences were in the decals, paint and accessories Give us some more pictures including the seat.
This is a fine bicycle, comfortable to ride and a treat to look at. The crank and rear hub look original as is the feather chain guard. Many manufacturers changed from the old skip tooth sprocket and chain design to what we use now around the end of WWI. Schwinn waited a few years, into the early to mid 1950's.
Some will advise you to leave it alone, others to restore it exactly to original and then ones like me who will say you should do what you want. You have to ask yourself then if you would ride it as a fairly heavy single speed bike and get some real use from it or would you be more likely to ride it if it had a motor. Your pick. There's no wrong choice. This is not so rare a bike that it would be a sacrilege to alter it. And you can add an engine without wrecking the bike... so that it could later be returned to how you have found it.
What a happy conundrum... lucky you!
Someday when I grow up I will probably lose interest in toys with wheels, but until then...