Usually you need a tool like this to tighten the barrel nut.
http://airsoftdepot.ca/catalog/produ...roducts_id=792
It really all depends on the kit though and if the previous owner butchered the parts together to get a cool look, found out it didn't work right, got frustrated and sold it.
For the optic the easiest will be just an extra rail riser to boost it one step.