just replace them with existing ones. umm...i thought the link took you straight to the product, but apparently not. the tires on those are REAL soft. but i can get u help with the mod itself:

http://www.rcmod.com/mod_pen_tires.html

for the tires and for the wheels:

http://www.rcmod.com/RCmod_fullsize.htm

it says fullsize wheels, but you do the same for the normal ones.