Engine knock?

I recently bought a 1995 Pontiac Grand Am Se with the 3.1 engine. When I bought that car it had a slight knock in the motor. I didn't think too much of it because they had changed lifters and the cam. After driving the car for 2 days and approximatly 350 miles, it died. Luckily I had a 1996 Chevy Monte Carlo that I totalled a little over a year and a half ago, and kept the motor out of. My brother changed the motor and everything seems fine. I am just curious if I should take the time and money to have someone rebuild the old motor as a back up. The motor from the monte carlo is presumably a replacement motor so actual mileage on that motor is unknown. Any advice is greatly appreciated.