I brought the Cat and sat it on my table but nobody asked me about it!
Anyway - All you need to fly with the Cat is a nice brushless system and a set of Lipos. Don't go overboard and slap in some insane 4.5t monster system though - the old girls belts can't cope with that and the fronts are hard to source (Schumacher stock plenty of rears though).
This works nicely. Its not really 540sized which means its not got the torque of other motors so is kinder in belts. Couple it with
this esc and you have a reliable combo that won't strain the car. You can run the 35A version but i find that overheats in the Cat.
Lipo wise what I did was just get one of
these and make a brace to hold it where the saddles used to mount. Its lighter than saddles so some weights will be needed. The one pack is all you need - amazingly you'll not use more an 1500mAhs in a race and you don't need to discharge Lipos. Just hook them to the charger and peak them back up. You'll need a Lipo compatible charger.