There's no way of telling if, or when, it's been rebuilt without taking off the head. If it were rebuilt and bored with oversized pistons installed, the oversize would be stamped on the top of the pistons.
If it has good oil pressure, good compression, no excessive oil burning (leaks don't count), and no bearing noise, I'd run it and be happy. Just remember that there are a lot of things that cause noise in an engine besides bearings, too. If there's a little smoke, I might try pulling the head and having the guides redone before thinking about a rebuild. By the way, blue smoke is oil, and black smoke is just a rich mixture. These little I6 engines have an impressive longevity when serviced regularly.
If it doesn't have enough power for you, there are things that can be done to help alleviate that, but it all takes money. A good place to start would be to make sure that the tune-up is to specs with new ignition components: points/condensor (or Pertronix ignition), distributor cap, rotor, plugs, wiring, timing, and a properly operating vacuum advance. If you want a lot more power, you might think of selling the car and buying a V8. It takes a LOT of money to get an I6 to where it will keep up with a stock V8. I like mine just the way it is, but each to his own. Hope this helped some!
