You can install a new lens, but it's a bit risky due to the loose specs around M12 lenses. M12 lenses focus by screwing them in and out, and some lens designs need to be screwed in further than others, which can make them hit the IR filter housing. It's hard to tell if this will be a problem without trying it out.
Likewise, if the new lens sticks out further or is bigger at the end, it can hit the dome or not fit the foam collar well. More light-sensitive lenses (F1.4 instead of F1.8, for instance) tend to have bigger lenses at the end of the barrel.
Hiks tend to have the lens held in by a spot of glue, which can be removed pretty easily with a bit of alcohol/brake cleaner/whatever and a tool for popping it loose.
A good source of reasonably priced decent quality lenses is m12lenses.com. You''d want the 1/3" MP board lenses:
http://www.m12lenses.com/category-s/64.htm.
6mm will get rid of your barrel distortion, but will zoom your image in further.
