Don't rush. Buy the Rewinder, and upgrade it always as soon, as possible.
With its help - and with good reflexes, of course - you can destroy the 
waves before they reach the end ball, and at the end of the stage you 
can collect more parts, then normally, because the end ball will be much 
closer to the start point. With a little practise, you can collect 100 
parts/level easily. Just don't be too selfish, because it's easy to ruin 
the whole thing with bad aiming.