If there will be similar physics checking utility as Relay Casper was/is, you can still use following tip to make legal physics changes to your cars. These modifications will take top power from your cars engine, making cars much more easily to handle (as easy as you want). You lose some speed in the long straights, but you can drive without errors, which means better results. And in slippery races (snow&ice), it may actually be faster in almost everywhere.
Look your calibration values of acceleration (gas) pedal. If the gap between min and max is e.g. 60 and you want to limit some highest last rpms from your engine, you can take 10% of the whole range (=6) from the value that you get when you press your gas pedal to the metal. So, if your full throttle value is 0 and min throttle 60, you need to set the value 6 steps outside of the range (to -6). 10% change in the range may take 20-25% from the peak power, making 240 hp to be maybe 190-200.
If the change slowed car too much (or made it too easy ;-)), then you just determine comfortable values for that stage or rally, just set the other value. This change can be done even during RT is running. Just switch from RT to DXTweak2 and change values and press apply and you have different car performance. If you find that it is possible to make the calibration limiting smaller, it is recommended to make, because the best possible results should be possible to make without limiting. But, if the car is (almost) impossible to handle, you can limit it as much as you want, so it is easier to handle the car in the beginning. With limiting of about one 3rd of the whole range, Ford Escort and Lancia Stratos are easiest cars in the RT. And they still have about the same power as standard Alfa Romeo or standard Fulvia.