Try running two of those resistors, connected in parallel. That will cut the resistance in half and double the current capacity. The motor doesn't care how it gets its current and/or voltage- - - -it just requires enough of each to run at the right speed. Placing the resistors inside the air box so the air from the fan blows over them will help to keep them cool. Just use a piece of non-conducting material like Bakelite to mount them onto the air duct.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!