you can stiffen up the front by putting a spacer on top of the spring
a piece of steel tube of suitable diameter will do the job
if its very soft now maybe start with a tube about 50mm long

the rear shock may have a threaded ring to adjust the preload
in which case just screw it up
if no adjuster then you can make a spacer
but you will need a lathe to make it
or go to a wrecker and try to find a stiffer spring