It's pretty simple. Any small block .15 or .18 will fit the motor mounts. You need an engine with either a pilot (SG) shaft or you need to cut down the threaded shaft to fit. AE sells a cut off nut that helps to do this.
Now the fun part, did you buy a pull-start kit or non? The side exaust parts are in the pull-start kit and the rear exhaust parts are on the non-pull-start kit. You can always buy the parts you need to go from one to the other.