1997 mitsubishi eclipse spyder help

have a 1997 eclipse spyder that i purchased friday from someone everything was fine with teh car. but monday put tags on and went to drive it and it is puttering like it is not gettting any horse power so i cahnged the spark plugs and the plug wires still nothing has helped. so my question is what else can it be also persdon i purchased car from states that it had 1 bad fuel injextor but he replace with a new one so any ones thoughts would help alopt i know nothing about cars

With a 13 year old car, the possibilities are…many.
Rather then just throwing parts at it, you need to do two things:

Bring ALL of its maintenance up to date.
Have a well-reputed independent mechanic diagnose the problem after doing all of the required maintenance.