The Medusa motor, being a 4 pole motor will outpull your old motor by lots, and run cooler doing it.
If you want to match your old top speed, go with a 18 tooth pinion.
If you want a higher top speed, go with a 20 tooth pinion. I'm sure the Medusa would have no problem tossing it around with this gearing. As long as you're still close to the stock vehicle weight.
Just in case, set your throttle endpoints to maximum and then recalibrate your speed control.
As "DOH!" as this sounds, it happens all the time. People think their setup is really slow but it turns out that they really didn't have the EPA adjusted all the way up.
I've got a 70mm in my Buggy and even with a 18T pinion the motor doesn't warm up at all (90-100 degrees at the most at the end of a run doing nothing but speed runs at full throttle). These motors like to be geared tall, I've seen 60mm MedUsa motors running 50+ MPH in an E-Revo on 4s lipo...
All I ever wanted was an honest weeks pay for an honest days work.