Is there anywhere that would allow you to do a runup? Not necessarily in the RWY or holding point area of course.
Otherwise your options are:
1. leave it full rich & lean after t/off
2. Lean it while on the move - possibly during a taxi along the runway. Be careful with this as it would be an understatement to say that it is easy to lose control!
3. Briefly set full power & lean while on the flight in to the airstrip. Obviously this needs to be done as close as practicable to the aistrip elevation & temp.
Note the mixture position & use something richer than this for take off.