Try a Medusa 60 series 2000 or 2200 kv.
http://www.rc-monster.com/brushlessmotors.php?pg=3 A much better engine at a v/good price as well. Mike also sells a heatsink clamp for the Medusa as well.
http://www.rc-monster.com/brushlessmotors.php?pg=4
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S