Back a few years ago I was crippled from a partial paralysis in my feet and lower legs (Guillane Barre Syndrome, similar to polio) and couldn't swing my leg over the back of the bike. So I started studying girls bikes (I have decided I prefer calling them "step through frames", as it sounds more manly, ya know) and settled on early sixties Schwinn Deluxe Americans, very similar to your very nice Acme Ladies Schwinn from the fifties. If anything there is more room on the Americans for a low engine mount. On one I have a pull start and had to go with a short armed pedal crank from a 20" bike (mine came from a Worksman folding bike... perfect!). I've just finished mounting heavy duty 2.125 wheels from bikeworldusa on both bikes and the ride is awesome, much better than the middleweight tires which came on the bikes. It is a tight fit, especially with middleweight fenders, but worth it. I also found front brake adapters for those early Schwinns so that I could mount a front caliper brake setup. Also a great improvement to these bikes. I like your gas tank in the back and would guess you have very little spill/leakage from it. I'll be on the lookout for those myself. One bike has a rear tank from thatdax... leaks at the cap no matter what, and the other has a square tank from a dead snowblower. Less leakage but not real good looking. On the other hand it holds a 1 1/2 gallons.
I like the kind of art deco look of these "step through" Schwinns, they're well made from when Schwinn was a proud company boasting "Chicago Made" and a selling point of the American model was that everything on that bike was made in the USA. Being able to step through and having the engine low enough on the frame to do that is a real plus and I think makes these good candidates as vintage motor bicycles. I was going to post pictures of mine and can't figure out how to do it. Mine are already resized by Picassa. If anyone can give me a tip on inserting photos I'd much appreciate it.
Silverbear