can i replace the lower counterweight?

As you know it's loose, have you tried just tightening the bolts that hold it in place?
If it's not badly cracked or broken, it may just need to be tightened.....
Tilt the machine onto it's back feet and either prop it up or have someone hold it for safety. Look at the block and if no concrete dust, rubble or small chippings are apparent on the floor it may just need tightening.
If it is broken, Just remove it carefully via the bolts, and the local store will have access to the company to order you a replacement.

Davymac
July 2009