You have two options:
1) Reinforce a standard aluminium suspension fork with sturdy torque arms on both sides. This will reduce the stress causing fracture in the aluminium wheel nut lugs. Torque arms can be homemade or bought:
You can get them from various sources, here are two:
http://www.ebikes.ca and
http://www.velectris.com/catalog/2) Use a steel suspension fork, they have much stronger wheel nut lugs. Schwinn use forks with steel bottom half (the part with the lugs on) on their Campus electric bicycle. Such forks can easily be found on the internet - google for "steel suspension fork". A bit of warning: Schwinn has a 180W motor on their bike, not 500W...
Good luck, and check your wheel often!
Perbear
