If you have the old 196 six then almost any newer engine will have more power and drive better. The 2V version of the 196 was rated 135 hp -- gross! That would be about 100-105 hp rated as modern engines are. A modern 2.5L four puts out more, but the 196 DOES have a lot of torque available right off idle -- it's a great "tractor" motor with 3.125" bore and 4.250" stroke. Doesn't rev up fast at all, but pulls right off idle. A modern EFI four with the accompanying modern trans would be a good power choice, something rated over 100 hp (you want more power... 100 hp four would be close to the same as the original engine). Since there are MANY I-4 and V-6 engines, not to mention V-8s, over 100 hp you're wide open for swaps. Remember that you will need the trans with the motor, and you might need to change the rear axle ratio to complement the engine and trans choice. Early Ranger 7.5" axles will fit, as well as 90-92 Ranger 8.8" axles (hard to find!). Later models are a bit wider and would be hard to fit in the American. The original axle is only 55" wide. The Ranger 7.5" is plenty tough for a 250-300 hp V-8 in a cruiser. If you plan on drag racing a lot you'll need the 8.8" though.
Oh well, that's enough on this old post! If you need any more opinions/advice let me know!