As far I can see, there is'nt mention any electrical upper limit for voltage or current.
Exept from that (but should still being taken in account) the thermal resistance will be the limited factor.
If you limit voltage to the motor's voltage ratio, you already have the windings resistance. As I can see, this is a bipolar motor, wich means it have a center connection and most probably four winding connections.
If some driver circuit fails and rated voltage is applied to all four windings at same time, the effect would be:
P = U^2/R = 4 windings * 6V^2/18,5ohms = 7,8W
So 7,8W is worst case scenario, provided that no higher voltage than rated voltage is used.