weight lifting isn't the best exercise for moto riding as its a 2D work out,but some is required just not a great deal, you need a 3D work like the rowing machines, medicine ball, jumping at max capacity in a vertical direction and horizontal, there is a site dedicated to motocross/enduro fitness, ill edit this post and add the link when i get a sec.
BUT they say the best training for riding is RIDING.......arm pump comes from holding on to tight, you need to relax your hands and grip with your knees pointing your toes in towards the motor(pigeon toed if you like), you also steer alot with your knees.
Link>
In-Season Training for Motocross - Racer X Virtual Trainer
I try to follow this with a few modification's