Toilet not flushing completely
Toilets don't really go "bad" unless they're cracked or rusted. Changing the internals might be a solution but I'd give one thing a try. I ran into this problem a few years ago and the solution was fairly simple. Get some good cleaning solution and maybe some of that CLR stuff - the chemicals that clean calcium deposits, rust, and lime scale. Apply it generously to the underside of the bowl rim (where the water flushes into the bowl). Let it sit and then scrub it really well. Use a small screwdriver (or even better - some chopsticks) to make sure all the holes are clear and fully open too. 

What might have happened is that the lime scale has built up and is partially obstructing the flow of water into the bowl. This can cause a "crappy" flush (punn intended). 

Beyond this, there's a number of other things that could be issues - clogged/restricted waste or vent pipes (does your sink gurgle when you flush?) or as previously mentioned - the toilet internals. 

Good luck.