What was the brand/model of that bike? Aluminum or steel. Did you paint the rims or did it come like that? The bike doesn't look familiar but it should make a great build.
Lay back seat posts are available but look for one with a support bracket. Many here have reported the cheap tubular ones bend. Another option if you know anyone in the metal working industry is to go with solid bar stock. Most seat posts are 1" and taper to 7/8" where the saddle mounts. A few minutes on a lathe and use heat to bend and you'll have a seat post that is totally bullet proof. Most cruiser style frames are a little short for most guys and getting the seat back 6 or 7 inches really improves the ride.
Good luck, keep us posted on your build.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"