Our toilet American Standard installed in 1988 is not flushing completely.If we were to change the entire guts of the tank would it run like new or should it be replaced since its 17 years old.
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.
If the toilet doesn't flush completely the tank may need more water or the "flapper" valve isn't letting all the water out of the tank.
With the tank lid off, watch the water and the flapper as you flush. If the flapper closes prematurely, it may need to have its height adjusted, or it may need to be replaced if it's torn or otherwise damaged.
If the flapper action looks good, the old guts should have a way to adjust the "full" water level. If it has a "float" on the end of a metal rod, slightly bending the rod up will increase the water level be raising the float. There may also be a set screw to raise the lever the float attaches to. Other value types will have a way to raise the water as well.
Replacing the guts could inadvertently raise the water level and also fix the issue, but you shouldn't have to replace anything if it fills properly and doesn't leak.