The 73/54 combination leaves only about 0.5mm of play left in the motor adjustment slot. I threw some long set screws in the motor and held it by hand in the picture below so accuracy wise it is not so good but qualitatively a 72 tooth spur is likely the smallest you could fit in. To get closer to the ratios your are talking about then you would need to get some evo4 gears and shave the tops of the drive gear so that you can get the e-clip on the shafts and use ring gears from a TB03 (takes transmission ration to 39/17, if I remember correctly). I ran that set up for a while I really liked it but I eventually put everything back to stock for TCS since the mod goes against the spirit of the TCS rules.