If your old cruiser is steel framed, you should be able to carefully spread the dropouts if needed. Procedures are on the late Sheldon Brown's website.
There are many options for internal gear hubs on EBay.
There is Shimano Nexus 7 and 8 speeds that offer a rear roller brake (modern drum brake). Some 3 and 4 speeds have a coaster brake. They are making them with disk brake mounts now, which would enable you to attach a motor sprocket.
Sturmey archer makes similar hubs with a variety of brake options.
Sachs (which has become SRAM) made 3 5 and 7 speed internal hubs.
SRAM still makes 8 speed hubs.
Except for the 3 speed hubs, they are expensive. That's why I suggested EBay.
