The Engine in the 72 I just bought runs good, but big loss of power

Here's my question---What's my best option? Rebuild the original, or buy a new one? It has the original 318, but I remember the 318 my dad's 71 had in it, and that little sucker was quick! The car reads 41,000 miles, so my guess is 141,000. Runs and drives good, but power is weak, so tell me the best way to go to save money.

Thanks, and Mopar to you!