On the outside of the engine there are some things....

1) better clutch setup.
The engines you mention are mostly used in simple RTR cars. The clutch will probably engage very soon. If you lighten the shoes and/or use a stronger spring(s) the clutch will engage at a higer RPM and so with more power.
2) exhaust.
With a RTR car there isa simple exhaust. With 2 stroke engines there is a lot of performance to find with the exhaust. I know TeamOrion has made some Option parts for Kyosho cars and also a good resonance exhaust for the Kyosho Fazer. Not sure if you will gain a lot but it should be better than what you have got now.
3) gears
In the past Formula-1 cars were alowed to change gear ratio's so for every track they optimized all gear ratio's to get the best out of it

And then there is the engine itself. Like with 2-stroke bikes you can modify the ports, carburator etc for more performance. There are some topics from Fyrstormer modding his HPI RS4 engine to reach 50mph, read them.