Hi there,
If you want to keep the model pretty much the same, then the easy improvement is to get a new magnet for the motor. You can get them on E-bay quite easily and you can usually get 3 for about £12. I did this to my old Triang Princess Elizabeth and it did make an improvement.
To fit all you do is open up the model and you'll see the X04 motor inside. Carefully unscrew the rear screw and lift the motor out. You can then slide the magnet out the back of the motor. Slide the new magnet in and screw it all together.
Hey presto! It should run quite a lot better...
Word of warning: Watch out for the new magnets, they're quite powerful and if two bash together with your finger in the way it *will* hurt!
Cheers,
Jason