There are many many chain drive systems here on ES; I would recommend looking thru the various build threads for "mid drive", "chain drive", "bottom bracket drive" (bb), "stoke monkey" (sm), "cyclone", etc. The search function in the forum can help you find some of these; others you may want to use the Site Search function in Google to locate.
If you look thru the CrazyBike2 link in my signature you'll also find some recycled-parts ways of doing it.
There are lots of possibilities, depending on your DIY skill level or desire, vs finding kits to do it.
Pictures and specifications of what you already have, as well as exactly what performance you expect out of it, and what loads you need to pull/carry over exactly what kind of terrain, would help others be able to suggest more specific options.