Clap Clap Switch Modifications
Probably the easiest way is to change the T1 to an NMOSFET to increase current switching capability. Since a lot of fans are designed for PCs and rated at 12V I would also recommend upping the supply to suit the fan. No other changes would be needed.

EDIT NOTE:
Have removed connection of pin 12 (carry out - output) from ground. (See EM Fields answer - nice catch sir!)