I measured the Corvette motors I had at around 19500 RPM at 12V, so you're running around 29k RPM at 18V. A fairly direct match would be these two motors:http://www.robotshop.com/en/rs-550-motor-19300rpm-12v-6249oz-in.htmlhttp://www.robotshop.com/en/banebots-rs-550-motor-12v-19300rpm.html
The specs are basically almost the same (one is $2.25 cheaper and has a rear shaft, which won't hurt anything; it also is in stock which is a bonus). Biggest gripe is that the front shaft is fairly short; the factory gear will press on and should be ok, but I haven't used this exact motor. You'll want to use some red loctite or other sleeve retainer and rough up the shaft surface. I'm fairly certain it has a fan based on the model name, but the pictures aren't clear; the Banebots one definitely does.
For 20% more speed (but likely even poorer longevity), you could try the 23000 RPM Nichibo motors from Jameco ($3/ea):https://www.jameco.com/z/KC3SFN-8021F-75-Nichibo-Taiwan-12-Volt-DC-Motor-19455-RPM_2258540.html
(or their ebay link here: Nichibo Ebay link
I was happy with their performance with Corvette 23T pinions on a PW Mustang, but only ran it at its rated 12V. Buy 4 and plan on replacing them
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors