I don't have enough time for the moment but i will seriously have a look on this car as soon as i can. I had never did this kind of modification before, but this 620r has so much potential that it worth give it a try. But right now i can't .
Tyres are not adaptated, gearbox not the good one, to much power
I made a copy paste and a few modifications from other cars (tyres, brake, gear box) and made a power lut from graphical found on the net .
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mod edit - External link removed
If you want a try i think this is closer from a 620r even if it's need a lot of improvement