Based on Longrat's experiment
http://www.oople.com/forums/showthread.php?p=840487
Basically I replaced the center shaft with the spur and pulley by a 5mm bore pulley mounted direct on the motorshaft of a 790KV out-runner motor.
A shorter belt was all i needed to finish it off.
Unfortunately the Kv and new gear ratio don't match well: way too slow.
Possible plans for the future are to find a bigger sized pulley, modify a 4mm bore pulley, or maybe even go for a higher Kv motor