you can get the webmin code from http://www.webmin.com/

as for examining and understanding the code, i suggest you take a look at the developer documentation

I'm not familiar with the code itself though I've used webmin a few times, so I can't help you with the specifics. I imagine that you can at least remove modules that aren't of interest to you, but how that works exactly I can't tell you.

update: http://www.webmin.com/modules-wbm.html seems of interest to your goal.