Really would depend on the engine. If it is strong and you are competitive in the class you want to run, then no rebuild necessary. If you aren't getting the results because the bike is too slow, do a compression test and leak-down test to see if you have some issues. You can then fix/rebuild. If you aren't getting results and it's not the bike's fault, fix that and leave the bike alone.

 

If you're just getting started racing, it's unlikely that the bike will be the reason you're slow... it's more than likely the driver.

Unless you're an experienced racer, I'd say that it's probably not necessary.

 

+1...spend $$$ on instruction, suspension, tires and track time.

 

my first racebike had some motorwork done to it. i would suggest against it. i wasted five years of learning how to race, while i was busy learning how to wrench. there's nothing like having to work on your bike EVERY single weekend just to race. imo, it wears out your mind and keeps you from getting better.

 

A stock motor will also be (theoretically) more reliable.

Be diligent with the recommended maintenance.

 

Don't touch the engine unless you really need to. That should be the last place for your money. Like others said; track days, suspension, tires and such is where your money should go to start off.
And as Zach stated, constant preventative maintenance is the key. Oil, brakes, chain and such.