Outside of old 10 speeds and 3 speeds, 26" wheels can be fitted with just about any decimal measurement 26" tire from 1.0 to 2.4. Cruiser frames have wide spacing, so they can fit any of these sizes with ease, but if you want fenders, you'll be limited to a 2.125 width.

Most cruisers use a one piece crank, which uses 1/2" pedal threads. These limit your pedal choices, but you should be able to get some you can fit straps to. You'll also want something with a predrilled fork so you can add a front brake.

Cruisers use a coaster brake hub: The cog is attached to a screw mechanism which engages the hub when turned clockwise, and the brakes when turned counterclockwise. There's no way to directly convert one of these hubs to fixed gear, so you'll need to have a new hub laced in, or just get a pre-built wheel. Again, since 26" wheels are pretty much standard across the board, you can get one that fits without having it be cruiser-specific. There is some range in rim sizes, and wider will make mounting a big tire easier, but it isn't absolutely necessary.