if your were racing id recommend a new car, but being your running it in parking lots throw some parts at it.
like i mentioned, the newer motor will be a good upgrade. to do this from the old g2d i believe you will need to change the small mount and use the solo style one. id put a set of front brakes on, i found front brakes make the car a lot more stable under braking. another thing id do is get an aluminum spur gear hub, the plastic one can wear out quick. the biggest thing to getting these cars working well is tires. being you run in parking lots id recommend soft fronts and extra soft rear.
post some pics of the car up and for us to have a look at. if we can see it it makes it easier to recommend things.